For several years, the faculty in the Mechanical Engineering program at Behrend have been using a modified mastery grading system in three sophomore-level courses: Statics, Strength of Materials, and Thermodynamics. In this system students are given up to three opportunities to solve each problem correctly. A solution must be essentially correct (minor errors only) in order for points to be awarded, otherwise a retake is required on a similar problem with the points awarded for a correct solution reduced with each retake. Although the effectiveness of this system has been evaluated qualitatively through student and faculty opinion, there has been no performance-based evaluation of the difference in student learning between the mastery system and the traditional partial credit system. This paper describes the results of a comparative study conducted during the spring 2015 semester on four sections of Strength of Materials (SOM). Students in each section (117 total students) of the SOM class were divided into a "mastery" group or a "partial credit" group such that there was no statistical difference between the GPA's of the two groups. Both groups took the same five initial exams during the semester, with the mastery group exams graded as described above and the partial credit group exams graded using a partial credit system. The partial credit group was not able to retake any of the exams. All students took a common final exam, which was graded on a partial credit basis, and the results of the two groups were compared. The final exam consisted of ten concept questions, five fundamentals problems, and three typical exam problems with all problems graded using partial credit. The results of this study showed that there was no significant difference in the total exam score between the two groups, nor was there a difference on the concept questions, fundamentals problems, and the first two typical exam problems. The students in the mastery group did perform better on the third typical exam problem (p-value 0.03). Based on these results it appears that mastery grading does not improve student comprehension of fundamental concepts, but that repeated practice does make them better able to solve specific problems. Sixty-two of these students were enrolled in a course for the fall 2015 semester which requires SOM as a prerequisite. The students were fortuitously evenly distributed as to GPA and the exam method from the previous course. During the first week of the fall semester, a prerequisite quiz consisting of the concept questions and fundamentals problems from the spring SOM final was administered, and the results from the two groups were compared. Based on this study, there is not enough evidence to indicate that student recall of material is impacted by the grading system.