This paper analyses the epistemic criteria that pre-service teachers consider when assessing the quality of a scientific investigation about friction force. The participants were 71 pre-service primary teachers working in 18 small groups of three or four participants (N = 18), who were enrolled in a science teaching methods course that addressed inquiry, nature of science and epistemology. They had no previous experience in planning scientific investigations and assessing. In the context of studying speed, the task consisted of investigating which material, among cardboard, plastic, cork or lacquered wood, would be the most appropriate to build a racetrack for toy cars. For data analysis, students' written products related to planning, carrying out and assessing the investigation were collected. The main findings highlighted a great number of criteria provided by the small groups for assessing the quality of the investigation, but little use of them in the assessments. Implications for promoting epistemic knowledge in inquiry environments are discussed.