I have developed a photon number resolving detector with superconducting transition edge sensors (TESs). The TES is a kind of calorimeters measuring the energy of incident photons as the increase of phonon temperature in a thin film absorber. The temperature rise also affects an electron temperature in the TES film resulting in the TES resistance change. The resistance change is proportional to the energy of incident photons. I have fabricated the TES devices with a thin film titanium superconductor embedded in an optical cavity to enhance the quantum efficiency. The titanium film has a relatively high transition temperature around 400 mK. With the illumination of weak coherent light pulses the fabricated device exhibited high quantum efficiency, fast response, and clear photon number discrimination up to 6 photons. TES-based photon number resolving detectors are quite promising for improving the performances of quantum communication and quantum optical radiometry.