"The Political Office deems it necessary to carry out the sixth Party Congress, and we will propose it take place in the second semester of next year," Castro said.
Representatives of about 900,000 party activists will "confront the challenges of the future" and "as Comrade Fidel says, guarantee the continuity of the Revolution when its historic leaders are no more," he said.
Feb 24: Raul Castro appointed new Cuban president.
Feb 29: Raul Castro signs human rights agreements rejected by Fidel.
Mar 13: Government allows sale of computers and DVD players for the first time.
Mar 28: Cuba's government permits the unrestricted use of mobile telephones for the first time.
Mar 31: Tourism ministry allows Cubans with enough money to stay in luxury hotels and rent cars.
Apr 2: Cuba begins lending unused state land to private farmers as part of efforts to boost output.
Apr 11: Raul issues decree allowing Cubans to acquire titles to housing they currently rent from the state.
Apr 27: President issues decree raising state pensions by up to 20 per cent to $9.50 per month.
Apr 29: Prisoners facing death penalty have their sentences commuted to imprisonment
Source: Media reports
Cuba's last Communist Party congress took place in 1997 and the following one in 2002 was postponed without any official explanation.
Raul Castro officially took over as head of state in February after his ailing brother Fidel, who ruled the island for nearly half a century, retired as the country's president due to health reasons.
Castro said an official announcement with a precise date for the congress would be made "at the appropriate time".
By law, an announcement must be made within six months of the meeting.
Castro also warned that the result of the forthcoming US presidential elections could have an impact on Cuba.
Castro said: "If the extreme right in the United States succeeds in imposing itself again in elections in November ... the world climate of instability and violence" will have a direct affect on Cuba.
The congress is also held to elect the 113 members of the Communist Party's central committee, 12 members of the secretariat and 21 members of the its political office.
In separate elections, the congress will also choose the first and second secretaries of the Communist Party, posts that have consistently gone to Fidel and Raul Castro ever since 1965.
The meeting will also decide what role, if any, Fidel Castro will hold in the party.
Juan Jacomino, a correspondent for Global Radio News, told Al Jazeera that the upcoming congress will not necessarily results in a shift from the socialist ideology of the country.
However some social and economic developments were already taking place in Cuba, such as giving people land to produce more food - the country imports much of its foodstuffs - and permitting Cubans to own mobile phones and DVDs, he adds.
Castro said the decision to commute the death sentences of an unspecified number of inmates was "a sovereign act in accordance with the humanitarian and ethical conduct" of the country, and not taken under international pressure.
Some inmates are set to have their death sentence reduced to a 30-year jail term while others will serve life in prison.
Castro did not indicate who or how many inmates would be affected by the move.
However, Raul said capital punishment would not be taken out of the penal code.
"We can't disarm ourselves before an empire [the US] that doesn't stop harassing and attacking us," he said.
Cuba's last executions were carried out on April 11, 2003, when three men faced a firing squad for hijacking a boat with 50 people on board and forcing the crew to take them to Miami, Florida.
The Human Rights and National Reconciliation Committee of Cuba estimates that there are between 40 to 50 people on Cuba's death row.