In this paper the deeply thermo-stable characteristics of telescopic resonators with G parameters satisfied the condition G1G2 = l/2 are theoretically discussed in detail. An exact expression for the telescope defocusing which can compensate thermal disturbance is derived analytically. It has been shown that the simplified approximation formulae of the defoousing in Refs. [4] [5] can be easily deduced from this exact formulation if the appropriate assumptions are adopted. Computer simulation and numerical calculation are carried out to confirm our theoretical results and the curves which describe the variation of beam sized with telescope-resonator parameters are given.