The novel unsteady mass (and momentum-) balance equations as well as the resistance to the initial velocity of a Newtonian fluid were proposed as rule of thumbs to predict the direction of gas flow in gas assisted injection molding (GAIM). Such configuration was adopted as that a pipe was connected vertically to a cavity formed between two parallel plates and gas was injected at the point above where two cavities filled with melt polymer resin were connected. Upon performing the simulation with various thickness of a cavity formed between two parallel plates and fixed diameter of a pipe using commercial software (MOLDFLOW) the results of simulation were compared with those of not only the ratios of the resistances to the initial velocity but also predicted-unsteady trajectories of gas penetration in GAIM by the suggested novel unsteady mass (and momentum-) balance equations to check the precision of predicted trajectories by these suggested flow models as well as the consistency of their predicted directions. In case of such a heterogeneous geometry as the combination of both a pipe and the cavity between two square flat plates (SFP), once the initial direction of most gas entered into cavities was determined to the side of two SFP since the value of the ratio of resistances became less than unity, the ultimate gas flow direction was decisively determined to that side due to geometrical reasons. However, in the reverse or the other cases, one should compare the effect of the ratio of resistances with the geometric effect of a fan-shaped cavity together with the loss of moving mass of melt polymer due to the formation of coated layer at the surface of the mold, which was treated in the unsteady model-predictions of the length of gas penetration using the proposed novel flow models. Consequently the values from the proposed novel flow models were excellently consistent with those from the simulation results while those of the ratio of resistances were fairly consistent with those from the simulation-results.