This study aims to determine the effectiveness of the implementation of guided discovery learning with virtual lab: Physics Education Technology (PhET) simulation in reducing students' misconceptions in mechanical wave physics. The design of this study was pre-experimental design with One Group Pretest-Posttest Design. This research was conducted in Mojokerto, East Java, Indonesia. Thirty five 11th grade high school students and one teacher participated in this study. This study applied certainty of response index (CRI) with Two-tier Diagnostic Test to determine students’ misconception. The results show that in the initial tests the 3 highest students’ misconceptions occur on the concepts: wavelength, wave velocity and the nature of the wave. But the implementation of guided discovery learning able to reduce students' misconceptions significantly. It based on the gain analysis that had been done (<g> = -1,13)