The performance of the side-pumped plane-plane asymmetric resonator, which comprises two identical diode side-pumped Nd:YAG laser heads arranged near the full reflector mirror, is theoretically and experimentally investigated. Theory analysis shows that, when the resonator operates at the boundary of the stable region, the fundamental mode diameter in the rods quickly increases, which leads to the laser beam quality quickly improving while maintaining a higher output from the resonator. Because the performance of this plane-plane asymmetric resonator operating at the boundary of the stable region is similar to a near hemispherical resonator design, we name such resonator design as thermally determined near hemispherical resonator. Comparing with the design of a symmetric plane-plane resonator operating at the boundary of stable region, thermally determined near hemispherical resonator design can shorten the cavity length and is suitable to low and middle power diode side-pumped laser. The performances of thermally determined near hemispherical resonator for cavity length of 970 mm, 820 mm and 770 mm, at a repetition rate of 100 Hz, are experimentally investigated. As expected, for above three cavity length, the beam quality all improves quickly and the output power saturates, once the pump power exceed certain value. For the 970 mm cavity length, under the total pump energy of 1.35 J, laser output with a pulse energy of 220 mJ and a beam quality of M<sup>2</sup>=1.28 is obtained.