Abstract This study aims to improve problem solving skill of Physics Education students in ULM Banjarmasin, Indonesia. Problem solving is part of 21st century skills which is recommended by Kerangka Kurikulum Nasional Indonesia (Indonesian National Curriculum Framework). This study starts by operationalizing the direct instruction model syntax and including the argumentation strategy in form of teaching material, then implementing the strategy using a valid teaching material. In order to see the effectivity of learning strategy, the implementation is done to 92 Physics teacher students in Faculty of Teacher Training and Education Lambung Mangkurat University who are registered in Thermodynamics module as the research subject. Pretest and posttest are conducted to gain data of strategy effectivity in improving students’ problem solving skill by using the analysis of normalized gain score. Problem solving skill test shows that the pretest average score of 28.89 is increased to 71.74 in the posttest with N-gain of 0.60 and it includes in medium category. Based on the research result evaluation, it is concluded that students’ skill in Physics problem solving in Thermodynamics on the topic of kinetic theory of gas can be improved by implementing the argumentation strategy in direct instruction model. Keywords: problem solving; direct instruction; argumentation strategy