A simulation study of VLF triggered emissions has been carried out in order to investigate quantitative changes of the velocity distribution function of resonant electrons caused by a combined action of nonlinear phasetrapping and geomagnetic inhomogeneity. It is a counter-part of the an analytic theory proposed by Roux and Pellat. The result basically supports their idea which is based on a simplified assumption. It also gives detailed information of more complicated behavior of both trapped and untrapped particles. It is pointed out that the contribution by electrons which are trapped in the midst of the whistler wave train is as important as that by electrons trapped at the wave-front. It is further revealed that deformation of the velocity distribution function is caused not only by trapped electrons but also by untrapped electrons perturbed by a whistler triggering wave. The well-known characteristics of pulse length dependence of rising or falling tone characteristics of triggered emissions is interpreted by the present simulation result.