Day Lewis, who co-starred with Postlethwaite in the movie "In the Name of the Father," led the addresses at a London church Wednesday and sang to his widow Jacqui.
He spoke of their days training together at the Old Vic Theatre School in Bristol, where his first professional assignment was as Postlethwaite's understudy.
Other guests at the service included British veteran actors Michael Gambon and Julie Walters.
Postlethwaite died at 64 years old last month after a long fight with cancer. His films included "The Usual Suspects," ''Inception," and "The Shipping News."