Water assisted injection molding technology has gradually become an important process for making hollow plastic parts, due to its light weight, relatively lower resin cost per part, faster cycle time, and its flexibility in the design and manufacture of plastic parts. Study of the temperature profiles in molded parts is of fundamental importance to a more complete understanding of many complicated phenomena. This report presents a novel experimental setup which allows the in-situ measurement of three-dimensional temperature field in the depth of molded parts throughout the molding cycles. A specific mold equipped with tubular needles for guiding embedded micro-thermocouples was designed and manufactured. Experiments were carried out on a lab developed water assisted injection-molding system, which included an injection molding machine, a water pump, a water injection pin, a water tank equipped with a temperature regulator, and a control circuit. The resin used was semi-crystalline polypropylene. The in-cavity temperature of the polymeric materials during the molding cycle was measured. In addition, an unsteady-state, nonlinear heat transfer model of water assisted injection molding has been proposed to numerically simulate the in-depth temperature profiles. It is shown that the numerical predictions are in good agreement with experimental data. Experimental investigation and numerical simulations of a water assisted injection molding cooling process will provide an improved understanding of the influence of water related parameters on the cooling process of water assisted injection molded parts.