The thickness dependence of the anisotropy in thermal expansion of poly(pyromellitic dianhydride oxydianiline) (PMDA-ODA) with flexible chains and poly(biphenylene tetracarboxylic dianhydride p-phenylene diamine) (BPDA-PDA) ranging from 2.5 to 20 mu m has been investigated. The lateral (in-plane) coefficient of thermal expansion (CTE) of PMDA-ODA and BPDA-PDA increases with increasing film thickness; in contrast, the vertical (out-of-plane) CTE of PMDA-ODA and BPDA-PDA decreases with increasing film thickness. The vertical CTE of PMDA-PDA is 4-7 times larger than the lateral CTE. The vertical CTE of BPDA-PDA is 14-20 times larger than the lateral CTE. It is believed that the thickness dependence of the anisotropy in CTE is caused by the in-plane orientation of the polyimide main chain. Since BPDA-PDA has a rigid-rod-like structure, this leads to a better in-plane orientation and a larger anisotropy in CTE than PMDA-ODA.