Abstract: There is an optimal length of gain medium in a laser diode (LD) end-pumped laser. The laser output performance will be worsened for too long or too short crystal. In order to study the influence of thermally induced losses on the optimal gain medium length, the method of combining theoretical analysis and experiments was adopted. Through theoretical analysis of the balance between gain and losses in the laser medium, the optimal gain length of Nd:GYSGG was calculated to be 7.8mm. With experiments of laser crystals of different lengths, the best laser output performance was conformed at 8mm. The maximum output energy is about 2.4mJ with pump energy of 7.6mJ at the repetition rate of 1kHz, corresponding to the optical efficiency of 31.6%. The results show that the investigation is significant for design and optimization of end-pumped Nd:GYSGG lasers.