"We voted against endorsing the government and ask... To be presented with another government," parliamentarian Ali al-Gaydi said.
Of the 104 lawmakers in attendance, 89 rejected the new government, he said.
A national unity government headed by businessman Fayez al-Sarraj and comprising 32 ministers was formed last week under a UN-sponsored accord signed last month.
Parliament member Al-Saliheen Abdelnabbi said the government had been rejected because its cabinet was too large.
He called for "a smaller government without this high number of ministries".