Intense excimer laser excitation of poly(methyl methacrylate) (PMMA) and polyimide films gives the transient expansion and the following contraction, which was directly measured as a function of nanosecond delay time during and after excitation. Depending on laser excitation condition, very rapid decay component and oscillatory behavior were observed in contraction and also phase transition is confirmed for PMMA. The behaviors are well interpreted by photothermal mechanism. In case of polyimide the expansion behavior is inconsistent with the simulation based on photothermal mechanism, on which we consider photochemical processes. New aspects of nonlinear polymer dynamics are presented and summarized.