Some factors affecting the temperature of laser crystal in Yb∶YAG thin disk lasers are studied. The theoretical and experimental results show that reducing thickness of the crystal and increasing the number of pump passes simultaneously are effective to reduce the temperature of crystal and to improve the output power and efficiency of the lasers. As a result, 15 W output power at 1.03 μm with a slope efficiency over 40% from a 0.35 mm thick Yb∶YAG disk laser is obtained.