This study aimed to investigate the beliefs and practices of group work assessment of secondary school teachers. For this reason, 213 teachers were selected randomly for a questionnaire survey. In addition, two teachers and five students were selected for interviews and focus group discussions at each site, respectively. A one-sample t-test was applied to analyze the data acquired through the questionnaire while the Pearson product-moment correlation was used to examine the relationship between the beliefs and practices of group work assessment. The data obtained through interviews and focus group discussion (FGD), were analyzed through thematic verbal descriptions. The findings showed favorable teachers' beliefs towards group work assessment. On the contrary, teachers' assessment practices were partial with focus on group product, with diminutive or no assessment of group process and individual contribution. Hence, Ethiopian Bureaus should conduct on- job training for teachers on the assessment of group work.