Service learning typically involves university students in teaching and learning activities for middle and high school students, however, measurement of university students' self-efficacy in science communication is still lacking. In this study, an instrument to measure university students' perceived self-efficacy in communicating science to middle and high school students was developed and validated using a sample of 104 university students (19 graduate students and 85 undergraduate students). The rating scale Rasch model and Winsteps computer program were used to analyze the students' responses to pilot and final revised instrument. The results have revealed that the final revised instrument which contains 20 items with four response categories is well-targeted and measures from this instrument are reasonably valid and reliable. Issues associated with using the instrument are also discussed.