Recent developments in students' assessment have required change in teachers' understanding and practices of assessment. Assessing student learning is an important skill that all teachers need to develop for effective teaching. However, it is a complex and difficult task to achieve because of the difficulties in changing teachers' traditional conceptions and practices related to assessment. Thus the study aims to investigate changes in prospective teachers' conceptions of assessment and their preference of assessment tasks after they completed an assessment course designed based on assessment for learning approach. The "Teachers' Conceptions of Assessment Scale" developed by Brown (2008) was used as pre/post-tests and an additional task was employed to elicit participants' preference of assessment tasks. Data collected from 118 prospective teachers, data of 89 prospective teachers, who completely responded to data collection tool, were analyzed to present findings. Results indicated that teachers' conceptions of assessment (school accountability, student accountability, improvement and irrelevance) did not significantly change while teachers' choices of assessment tasks significantly changed towards more use of alternative assessment tasks. The results showed that changing teachers' conceptions of assessment is complex and requires considering other factors such as experience and culture. The results also showed that assessment courses that were organized around assessment for learning approach encourage prospective teachers to choose and use more alternative assessment tasks.