Critical thinking has been one of the 21st-century skills consistently associated with students' future career advancement as a positive student outcome of STEM education. The aim of the study is to develop and validate science critical thinking skill instruments to assess the improvement in the subject of living organisms and force and friction through design-based STEM education. In this design-based research study, the student's modules were developed by the integrated STEM education principles involving the activities and worksheets in line with the frame of critical thinking approach. The kappa statistics for content validity, exploratory and confirmatory factor analysis for construct validity, and item and reliability analysis for the quality of items were used in the development stage of instruments. The results of these analyses endorsed the 15 two-tier item for each test of Living Things Critical Thinking (LTCT) and Measuring Force and Friction Critical Thinking (MFFCT) as unidimensional constructs to produce valid and reliable data to measure the fifth grade students' critical thinking skills in the related science content. Comparing the pre and post applications of instruments in the study group indicated that STEM modules improved the students' science critical thinking skills such as interpretation, analysis, and inference. In this respect, developing and validating instruments to assess the integrated critical thinking skills will contribute to the empirical examination of this construct within the context of school science learning. 1.1. The Theoretical Framework Critical thinking is the process of mentally acting on something by "making reasoned judgments" (Beyer, 1995, p.8). Facione (1990) defines critical thinking as "purposeful, selfregulatory judgment which results in interpretation, analysis, evaluation, and inference, as well as explanation of the evidential, conceptual, methodological, criteriological, or contextual considerations upon which that judgment is based" (p. 2). Thinking skills in education settings generally involve activities of comparing and contrasting, classifying, predicting, generating original ideas, cause and effect, decision making, uncovering assumptions, and determining the reliability of sources of information (Swartz, Costa, Beyer, Reagan, & Kallick, 2008). Critical thinking includes process skills of "analysing, evaluating, or synthesizing relevant information to form an argument or reach a conclusion supported with evidence" (Reynders, Lantz, Ruder, Stanford, & Cole, 2020, p.4). Critical thinking enables individuals to develop their way of thinking about any subject, content, or problem by skilfully handling thought-specific structures and assigning intellectual standards to them. Then, these individuals can use the principles that help them to improve their thinking while analysing and evaluating the problems or their thoughts (Gencer & Boran, 2017). In addition, critical thinking skills are required in the process of analysing possible solutions during the probl...