The lifetime of atoms and molecules can be measured by using modulated resonant single-mode laser field. A theoretical mode for this method is presented. It is shown that the laser induced fluorescence, a resonance flourescence is modulated at the same frequency, but phase shift from the exciting laser. The phase shift is mainly due to three processes (1) coherent excitation process characterized by transverse relaxation rate and line width of the laser, (2) population damping process characterized by the relaxation time by which the system to be in equilibrium state. The effect of these processea on measurement result is discuesed. The Doppler broading is considered also.