This study was conducted to examine the effectiveness of formative assessment techniques used by high school mathematics teachers in Jimma at CBTP and practicum zone sites. Specifically, the study sought to gain an understanding of the extent to which the teachers use different assessment techniques and to support both the learning and teaching process in mathematics. Questionnaire was the main data gathering instrument for this study. Thus, all mathematics teachers’ 94 were included in the sample by using availability sampling technique since they are few in number and 253 students were selected by systematic random sampling technique to fill in questionnaire. Class room observation and interview was also conducted to enrich the quantitative data. The data gathered through interview and class room observation was discussed in line with questionnaire. As a result, the main findings come out from this study were: teachers’ perception of formative assessment and its connectedness with students learning and achievement in mathematics is insufficient; assessment for learning techniques help to encourage the necessary classroom environment for effective learning is not good enough. Finally, to minimize and if possible to solve the problems, the following recommendations were drawn; teacher and school administrator assessment literacy was a prerequisite for successful formative assessment implementation and hence Zone Education Departments and the regions in collaboration with schools should give training for the teachers and school administrator, all the zones can take the following tangible steps now to begin to implement research-based formative assessment strategies in their schools and districts that can yield positive results.