Abstract: Based on the governing equation of filling stage, a continuous transient design sensitivity method was firstly developed and applied to injection molding.The sensitivity of pressure, temperature and velocity with respect to a design parameter was calculated.The numerical solution was based on a hybrid finite-element/finite-difference method, which was popular in filling simulation, to solve for the pressure, temperature and velocity sensitivity, and a control-volume method was adopted to obtain moving melt front sensitivity.The injection pressure sensitivity with respect to flow rate and melt temperature was obtained by using numerical methods.These results were found to agree reasonably well with corresponding numerical experiments.