The semiclassical equations which describe the behaviours of three-level lasers were derived. The method of nonlinear mechanics was used to treat the electromagnetic oscillations coupled with the dipole moments of molecules. It is shown that when the pumping power is larger than the critical value, the amplitude of output oscillations is stable (corresponding to a stable equilibrium point in phase space), only if the homogeneous line width of molecules (1/T2) is larger than the damping width of the resonan-tor (q). The appearance of instability of output oscillations is due to the effect of coherence. Furthermore, a stable limit cycle was shown to appear when the above mentioned equilibrium point becomes unstable as qT2 increases. Therefore, it seems true that there is no limit cycle near the equilibrium point when it is stable. These results may account for the instability of amplitude of output oscillations observed by some authors in ruby lasers, and should be checked in detail with gas lasers.