The expectancy-value model of motivation states that individuals' choice, persistence and performances are related to their beliefs about how much they value task. Despite the importance of "task value" in learning biology, lack of the instruments on task value for high school biology courses for practical use indicated requirement to develop an instrument. The purpose of this study is development of "task value" instrument for using in high school biology courses. The study was conducted on 189 ninth grade high school students by applying "task value" instrument, self-efficacy and test anxiety scales. To collect validity evidence, confirmatory, divergent and convergent validity analyses were performed while internal consistency was determined by Cronbach alpha reliability analysis. The results showed that scores on the instrument with 18 items loaded on three factors (importance, interest and utility) and Cronbach alpha values were 0.64 for importance aspect, 0.88 for interest aspect and 0.84 for utility aspect. Based on the findings, it can be suggested that the instrument can be used to determine current motivational status of the students and to predict future performances, efforts and persistence on biology tasks.