Affect has been identified as one of the important inputs in illuminating the mechanism of cognitive activities in mathematical problem solving (MPS). This study aimed to assess upper primary students' MPS-related affect with a focus on its differences across gender and grade-level among upper primary students. To do so, a simple Likert type scale was developed to measure students' MPS-related affect. Moreover, the study examined how this affective structure varies across grade levels and gender among upper primary students. The scale includes 5 subscales; Vision, Value, Enjoyment, Effort, and Self-efficacy regarding MPS. The responses of 725 upper primary students from different schools were considered to carry out the psychometric analysis and to demonstrate the scale's potential usefulness for measuring the MPS-related affective construct. The results revealed that girls and boys did not show any significant difference in the subscales, except for Vision of MPS, where girls report a higher dynamic (a non-static) vision than boys. When comparing grades, there were significant differences among these across variables. However, almost all variables (except Vision) show a decreasing trend, while Enjoyment and Effort have a wide range of changes along with grades. Ultimately, these differences need to be considered by mathematics teachers and curricula.