Investigation of the properties of the liquid crystalline polymer (LCP) retarder films, formed by photo-polymerization of a reactive LC mixture at various conditions, has shown that the alignment, the birefringence and the thermal stability of the LCP film are governed by the conversion of photo-polymerization (CP) of reactive LCs. The CP of the LCP film photo-polymerized in air has increased with the intensity of irradiation light, which reduces both the degree of in-plane alignment of LCs and the birefringence but gives the better thermal stability of the film. The photo-polymerization in an inert environment has induced the higher CP and the better thermal stability of the LCP film than the in-air polymerization.