Abstract:We gave the nonlinear theory of invers ionless lasing of frequency up-conversion in an open V-type system with a mon o chromatic incoherent pump.We studied the effects of the Rabi frequencies of the driving fields,the injection rates,the exit rate,the decay rates between atomic levels,as well as incoherent pump rate on the gain without inversion,dispersion and the population difference of the system,and discussed the stability of the g ain without inversion.