Assessment as learning (AaL) puts students at the centre of learning and is considered a powerful alternative assessment approach that can maximize student learning. How it can be applied as a pedagogical and assessment tool in writing classrooms, however, is not clearly understood. Research on the implementation of AaL as a unitary concept in L2 writing classrooms is scant, particularly at the primary level. To fill these gaps, the study investigates the implementation of AaL in two Hong Kong primary writing classrooms from teachers’ and students’ perspectives, focusing specifically on the perceived benefits and problems. Using data gathered from interviews with two participating teachers and six selected students, as well as classroom observations, the findings show that the teachers’ very first attempt to put AaL into practice reaped benefits for themselves and the students, but a few problems were noted too, which provide useful implications for practice and further research.