As a result of the enactment of the National Education Act B.E. 2542 in Thailand, teachers have been encouraged to employ various methods for students' learning assessment including student portfolio. Student portfolio is a collection of evidence that systematically reflects students' learning processes and their learning outcomes in various aspects. The purposes of this study were to examine teachers' misconceptions in implementing student portfolio assessment, and to compare teachers' current performance in implementing student portfolio assessment between teachers who had attended the training sessions concerning portfolio assessment and those who had not. Four hundred and fifty-four elementary school teachers were randomly sampled to be participants in this study. Questionnaires were employed to collect data on teachers' misconceptions about the principles of student portfolio assessment and the utilization of the results from student portfolio assessment, as well as their current performance in implementing four main steps of student portfolio assessment [i.e., (a) planning for portfolio assessment, (b) collecting created products, (c) selecting products and reflecting on selected products, and (d) revising and evaluating products]. Results revealed that, overall, teachers had misconceptions in nine concepts. Six concepts were about the principles of student portfolio assessment and three concepts were about the utilization of the results from student portfolio assessment. Performance, which consisted 1 Please send all paper correspondence to: Kamonwan Tangdhanakanond, Ph.D.