This paper presents the theoretical and experimental study of a slab laser. Analytic solutions of temperature and stress distributions in condition of CW and pulse pumping are given. We have analyzed and discussed the thermal distortion in the slab. Measurement and analysis of the thermal effect by using a 9 × 30 × 70 mm3 Nd:YAG slab were carried out. A high repetition rate zig-zag slab laser was put into operation with output energy per pulse at 50 Hz of about 600 mJ and average power of 30 watts.