A regenerative amplifier taking a thin disk Tisapphire crystal (2 mm thick) as gain medium is presented. The front face of crystal is anti-reflection (AR) coated at 532 nm and 750~850 nm, the other face is high reflection (HR) coated at 532 nm and 750~850 nm, and the total absorbing efficiency of the crystal is over 80% at 532 nm for double passes. In the regenerative cavity, the Tisapphire crystal also acts as a flat reflective mirror, which reduces the complexity of amplifier. The crystal is end-cooled to reduce the thermal effects and improve the beam quality of amplified laser pulses, as a result, the output energy of the amplified laser is about 5.2 mJ, the corresponding conversion efficiency is 11.5%, and the beam quality factor M2 is less than 1.2.