The rejection of Saidloo on Wednesday, as well as the nominees for the ministries of welfare, cooperatives and education, came after a large number of deputies in the Majlis argued the candidates were totally lacking in experience.
It was a minor blow for Ahmadinejad, a former Tehran mayor who was elected president on 24 June, given that the rest of his 21 mainly ultra-conservative nominees did make it through the confidence vote.
The Majlis has been controlled by right-wingers for over a year, after parliamentary polls that saw most reformist candidate barred from even standing.
Most of the objections related to the proposed cabinet were motivated by questions of competence and not politics, and the few reformist deputies in the assembly were largely silent.
Several deputies who spoke out against Saidloo asserted that the oil ministry in Iran, OPEC's number-two exporter, needed a much stronger figure at the helm.
Parliament speaker Gholam Ali Hadad-Adel asserted after the vote that parliament merely "wants to have a stronger government" and said the rejection of four ministers "is not a sign of non-cooperation between the Majlis and the government".
For the still-vacant ministerial posts, Ahmadinejad has three months to put forward new nominees to a confirmation vote.
Also on Wednesday, Ahmadinejad promised to offer new proposals soon for negotiations with Europe over the country's controversial nuclear programme.
France says talks with Iran over
nuclear programme are possible
The comments by Ahmadinejad suggest he wants to launch a new dialogue in hopes of persuading Europe to recognise Iran's right to enrich uranium.
A spokeswoman for the International Atomic Energy Agency, Melissa Fleming, had no immediate comment on the president's statement.
France's foreign minister said on Wednesday that the European Union still believes negotiations with Iran over its nuclear programme are possible, despite the EU's cancelling an 31 August meeting because of the resumption of reprocessing.