The purpose of this research was to develop and validate a new instrument, the "Mental State in Learning Environment Questionnaire" (MSLEQ), to assess student's mental state in a given learning environment. The MSLEQ has high internal consistency reliability values between 0.70-0.92 as well as good construct validity and predictive validity. After conducting a factor analysis, four main factors were extracted and were described as "emotion", "intention", "internal mental representation" and "external mental representation". The important feature of this study is the construction of an economical questionnaire on the mental state in a given learning environment. The questionnaire yielded important information that can be concretely applied to science teaching and learning.