One of the most important variables affecting middle school students' mathematics performance is motivation. Motivation is closely related with expectancy belief regarding the task and value attached to the task. Identification of which one or ones of the factors constituting motivation is more closely related to mathematics performance may help more effective mathematics planning process. In this context, the purpose of this study was to investigate the relationships among middle school students' task values concerning the math class, their expectancy perceptions and mathematics performance via a structural equation model. The study was conducted on 200 middle school students receiving their education in Konya provincial center. Fifty point five percent of the students (n=101) were female whereas 49.5 %. (n=99) were male. Twenty-nine percent of the students (n=58) were sixth graders, 35 % (n=70) were seventh graders and 36 % (n=72) were eighth graders. The Self and Task Perception Inventory in mathematics was used to identify the students' task values and expectancy perceptions concerning mathematics, while end-of-the-term math class scores were used to determine their mathematics performances. The data obtained were analyzed using the structural equation modeling. According to the results that were obtained, it was understood that expectancy beliefs, task difficulty and intrinsic interest value were the most effective variables on mathematics performance. Moreover, findings of this study show that students with high expectancy perceptions in mathematics who derived pleasure from dealing with mathematics and had less difficulty in mathematics had higher mathematics achievements. The findings that were obtained were discussed in light of theoretical explanations.