Self-focussing by a Kerr nonlinearity in combination with an intracavity aperture creates a power-dependent amplitude modulation in Ti:sapphire lasers.It has been used as a method of self-mode-locking for generating femtosecond pulse.The general features of this mode-locking technique is described and a new,simple model is developed for the optimization of Kerr-lens-mode-locking performance by taking account of gain modulation created by pump beam in conjuction with laser beam.Theoretical prediction is confirmed by experimental measurement.