Teacher-oriented learning, thus the teacher's ignorance of the initial knowledge of mathematics, students allegedly resulted in a lack of student mathematics learning outcomes. The Problem Based Learning (PBL) model places students at the center of learning which builds creativity, challenging conditions, contextual, and diverse student learning experiences to solve problems related to the material. The purpose of this study was to see the effect of the PBL model on mathematics learning outcomes in terms of initial knowledge. This research is a quasi-experimental study with a Complete Randomized Block Design. The sample of this study was Class 8A students as the experimental class and Class 8B as the control class at SMP Negeri 2 Belang. Each class is taken by 16 students, divided by high and low initial knowledge. The results showed there was an influence of learning models on mathematics learning outcomes, there was an influence of initial knowledge on mathematics learning outcomes, there was an interaction between learning models and initial knowledge on mathematics learning outcomes, there were no significant differences in learning outcomes between classes with PBL models and classes with models conventional for students with high initial knowledge, there are significant differences in learning outcomes between classes with PBL models and classes with conventional models for students with low initial knowledge, there are no significant differences in learning outcomes between students with high initial knowledge and students with low initial knowledge in the classroom with the PBL model, and there are significant differences in learning outcomes between students with high initial knowledge and students with low initial knowledge in the classroom with conventional models.