Scientific knowledge is one of the expected direct impacts in the Scientific Inquiry learning model (Joyce, 2009), which means that Scientific Knowledge is the learning outcome that is to be achieved in learning. Scientific knowledge is the result of student physics learning that is concerned with strengthening cognitive structures in understanding, mastering and applying physics concepts so students can solve physics problems from simple to complex. As for the purpose of this study are: 1. To know whether or not there are differences Scientific Knowledge of Physics between students taught using the Scientific Inquiry learning model and conventional learning 2. To find out whether or not there are differences in Scientific Knowledge of Physics caused by students 'Formal Reasoning 3. To find out the interaction between the learning model and Formal Reasoning in increasing students' Scientific Knowledge? As for research design in this study using different analysis for the two dependent variables. Variables are bound to Scientific Knowledge using analysis of variance or 2x2 ANOVA. The research instrument used was in the form of a description for the scientific knowledge test, and multiple choice forms on the students' formal reasoning test. The conclusion of this study was 1. The ability of scientific knowledge of students to use scientific inquiring learning was better than the scientific knowledge ability of students using expository learning models, The ability of scientific knowledge of students in the formal reasoning group is above average better than the scientific knowledge ability of students in the formal reasoning group below the average, 3. There is an interaction between the learning model and formal reasoning in improving students' scientific knowledge. The scientific knowledge learning outcomes of students taught through the scientific learning model in the formal reasoning group above average and formal reasoning are below the higher average compared to the results of scientific knowledge learning students taught through conventional learning in the formal reasoning group above average and in the formal reasoning group below average.