ABSTRACTTraditional models for determining economic production quantity assume a constant production rate. One result of this assumption is that the number of units produced in a given period is constant. The constant production rate assumption is not valid whenever an operator begins production of a new product, changes to a new machine and restarts after a long break, or implements a new production process. In such situations, the learning effect on production planning and inventory management cannot be ignored. The conventional Wright's learning curve is a curve that solely depicts productivity increasing at a constant percentage as a cumulative number of units are produced. Thus, in addition to the productivity learning process, the quality learning process should also be considered. With the recent development of learning curves that simultaneously take into account quality and productivity, a modified production inventory model has been built to periodically determine the optimal lot size for a finite plan horizon. The productivity considered is the unit production time, while the quality concerned is the unit standard deviation of the production quality level. For the purposes of explanation and comparison, four cases are introduced in this research, and examples are provided for demonstration.