Detlev Mehlis, who began his investigation on 17 June, arrived in Damascus on Monday.
His visit comes after the 30 August arrests of four Lebanese generals who controlled security in Lebanon at the time of al-Hariri's death.
All four are suspected of having close links to the Syrian security services. Two other Lebanese politicians with close Syrian links have also been questioned.
But Damascus denies any links to the 14 February attacks and promises complete cooperation with the investigation.
No Syrian suspects have yet been identified.
Mehlis has accused Syria of slowing down his inquiry through a lack of cooperation, including ignoring requests to interview Syrian officials and handing over documents.
The chief investigator is expected to meet Riyadh Dawoody, a legal adviser and member of the Syrian Foreign Ministry's political committee to discuss how to conduct interviews in Syria.
He also reportedly wants to interview the country's last intelligence chief in Lebanon, Brigadier-General Rustum Ghazale, as well as two of his aides and Syrian Interior Minister Ghazi Kenaan - who was intelligence chief in charge of Lebanon until five years ago.
His inquiry may eventually include a meeting with President Bashar al-Assad, who cancelled a visit to the United Nations for the world leaders' summit this week amid US efforts to isolate him and international pressure to cooperate in the investigation.
Lebanese President Emile Lahoud, however, left for the New York summit, despite calls to remain here as the probe continues.