This article describes the development and validation of an instrument that can be used for content analysis of inquiry-based tasks. According to the theories of educational evaluation and qualities of inquiry, four essential functions that inquiry-based tasks should serve are defined: (1) assisting in the construction of understandings about scientific concepts, (2) providing students opportunities to use inquiry process skills, (3) being conducive to establishing understandings about scientific inquiry, and (4) giving students opportunities to develop higher order thinking skills. An instrument--the Inquiry-Based Tasks Analysis Inventory (ITAI)--was developed to judge whether inquiry-based tasks perform these functions well. To test the reliability and validity of the ITAI, 4 faculty members were invited to use the ITAI to collect data from 53 inquiry-based tasks in the 3 most widely adopted senior secondary biology textbooks in Mainland China. The results indicate that (1) the inter-rater reliability reached 87.7%, (2) the grading criteria have high discriminant validity, (3) the items possess high convergent validity, and (4) the Cronbach's alpha reliability coefficient reached 0.792. The study concludes that the ITAI is valid and reliable. Because of its solid foundations in theoretical and empirical argumentation, the ITAI is trustworthy.