This study investigates the effect on conceptual change of instructional materials about "decimal fractions" developed by adopting 4 stage constructivist learning model. The study was conducted with 30 students 6th grade at a public elementary school in Trabzon during the spring term of 2008-2009 school year. Case study method was used in the study. The clinical interviews were made with 6 students just before the application and one month later. When the students' responses to the interview questions after the application were investigated, it was identified that the number of students' responses containing misconceptions was decreased at the end of the study. [Paper presented at the World Conference on Educational Sciences (WCES) 2011.]