This study aimed to determine the quality of the questions and the effectiveness of the two-tier multiple-choice diagnostic test, assisted by the CRI (certainty of response index), to identify students' misconceptions about Newton's law of motion. The type of research used is research and development (R&D) research using the Tessmer development model, which consists of 4 stages:  preliminary, self-evaluation, prototyping, and field tests. The trial subjects in this study were 30 students. The instruments used to collect data in this study were tests and questionnaires. The results of this study indicate that the developed diagnostic test meets the valid criteria due to a Content Validity Index value of 1.00 in a very appropriate category. The student response questionnaire met the "achieved" criteria because more than 50% of students responded positively. The results of the reliability test are 0.730 in the high category. The difficulty level of the questions is in the range of 0.48–0.60 in the medium category. The discriminatory power of questions ranges from DP > 0.2 with sufficient category. For the diagnostic test, the average percentage of students who understand the concept is 45.33%, the percentage of students who experience misconceptions is 23.50%, and the percentage of students who do not understand the concept is 31.16%.