A new photographic method was developed to measure the evolution of shock wave front velocity in polymers continuously with time. This kind of experiment is motivated by our previous studies which showed that the Hugoniot curves of two kinds of polyethylene specimens have very peculiar shape in the vicinity of the particle velocity around 150 m/s. We observed the shock velocity history in PMMA and two kinds of PE specimens especially in the vicinity of 150 m/s. Shock velocity in PMMA is found to be almost constant within the experimental conditions tested. While in PE specimens it becomes slower with the propagation distance. This tendency depends strongly on shock strength as well as on the difference in materials. It is noticeable that this unsteadiness of shock velocity was found to be different even in PE specimens with different molecular weight. ©2000 American Institute of Physics.