The main concern in designing of multi-cavity molds is flow balance between cavities. Any departure in flow balancing of the cavities can resulte difficulties in processing and quality of injected parts. In this paper, flow balance in a two-cavity plastic injection mold with different sizes (or called family-cavity mold) was investigated. Moldflow software was implemented to predict the filling phase through the cavities. Diameters of runners related to each cavity were adjusted to attain a balanced flow. Evaluating the flow balance was conducted by injection molding as short-shot and measuring the weight of each cavity. A high density polyethylene (HDPE) was applied as plastic material in this research. Good agreement was observed between experimental and simulation results. Moreover, in this paper one of the runners could be resized while injection molding via an insert located in the mold. The effect of flow balance on the tensile properties of the injection molded specimens was investigated. The results indicated that the parts obtained from the balanced mold exhibit a higher tensile strength and elongation at break up to 14% and 18%, respectively. The dimensions of injected parts were measured. It was found that there are not any differences between the shrinkage of specimens obtained by balanced and unbalanced mold.