La0.5Sr0.5MnO3 nano crystalline was synthesized by sol-gel auto-combustion method using citric acid as the chelating agents. Some analysis methods, which consisted of FT-IR, XRD, DSC-TG, wave-guide, were used to character the gel and the products after calcining. It was shown that metal ions and carboxyl were coordinated by the orientated manner. The phenomenon of melt, combustion and forming crystal for gel were observed in the process of thermo-decomposition, the nano crystalline had a dielectric loss in the frequency range of measurement. With the increasing of thickness, the reflectivity of perovskite-type nano crystalline increased and absorption peak shifted from high frequency to low frequency.