A two fluid model has been developed to calculate the critical two phase flow rates for initially subcooled and saturated liquid. This model considers vapor and liquid superheat, and slip between phases. This utilizes bubble growth with linear temperature decay for vapor phase, and bubble generation with heterogeneity factor obtained by empirically. This model requires some empiricisms for parameters characterizing the density of nucleation sites in the liquid and certain pressure gradient above which vapor follows linear temperature decay, and initial bubble radius. Comparisons between measured values and the predicted values of the present model show a good agreement regardless of test geometry.