The stress in films of semirigid and rigid polyimides (PIs) on silicon (Si) substrate has been measured in situ during curing using a newly modified bending beam apparatus. By using the apparatus, the onset of residual stress in the initially solvent-rich films has also been investigated. The stress characteristics of the PI films are strongly ramp rate dependent. Different ramp rates result in markedly different stress patterns, thermo-mechanical properties, and film morphologies. The residual stress at room temperature after curing is seat tered around 23-31 MPa for the films of pyromellitic dianhydride-4,4’-oxydianiline (PMDA-ODA). For pyromellitic dianhydride-p-phenylenediamine (PMDA-PDA), it systematically increases from -6 to 28 MPa for ramp rate increasing from 0.5 to 10 degrees C/min. The residual stress is very low in the slowly cured PMDA-PDA films. However, these films have gone through a very high-stress transient state during curing. The maximum transient stress is comparatively higher in the films of PMDA-PDA than in PMDA-ODA. Slowly cured PMDA-PDA films exhibit high structural ordering, high in-plane anisotropy, and low coefficient of thermal expansion.