Abstract: In this paper,starting from the rate equation of a quasi three level system,the rate equation of diode pumped solid state lasers has been derived.A method for calculating the output power in longitudinally diode pumped CW lasers has been proposed,which is represented as a function of the two parameters:the active medium length and transmissivity.The optimal design procedure has been found.Numerical calculations have shown a good consistency with our experimental result and the data in Refs. 3 and 4.