All students come to the classroom with their own ideas about a number of science phenomena. In the classroom setting, English language learners may have ideas about heat and temperature that present additional challenges for teachers. In fact, their ideas can stem from many different influences and English language learners (ELL), in particular, may have misconceptions about topics and language barriers, or misconceptions, that are culturally or language-based (Lee, 2001). This action research thesis was performed to explore the research questions: How did my use of formative assessment affect ELL students' misconceptions about heat?, How did my use of formative assessment uncover students' misconceptions about heat? Formative assessments were used in the classroom to uncover students' misconceptions about heat and temperature. The students performed labs based on the formative assessment activity sheets. The students answer before and after questions related to the labs. Data were collected and analyzed to examine changes in ELL students' conceptions of heat and temperature. Data showed that some ELL students changed their ideas about heat and temperature but other misconceptions remained. Time allotted to instruction and alignment of laboratory activities with formative assessments need to be further explored to address changing students' ideas about heat and temperature.