This mixed-method research aimed to determine the influence of lesson study on planning, instruction, and self-confidence of pre-service Grade 8 mathematics teachers. Ten pre-service teachers coming from a certain state college in Iloilo City were purposively chosen to be the participants of the study. The ten participants were grouped into two. Conducted from January to March 2014, this research utilized the reliability tested and validated instruments, namely: Lesson Study Instructional Effectiveness Rubric, Lesson Study Instructional Planning Questionnaire, Mathematics Teacher Self-Confidence Survey, Field Notes and Video Analysis, Pre-service Teacher's Weekly Reflections, and Semi-structured Interview, The statistical tools employed were mean, standard deviation, Mann-Whitney U Test, and Wilcoxon Signed Ranks Test. Results revealed that the lesson study process had a very high influence on the instructional planning of the pre-service teachers teaching grade 8 mathematics. The collaborative planning demonstrated improvement of planning skills of the pre-service teachers, open mindedness for corrections, and opportunity for further collaboration. In collaboratively planning the lesson, the pre-service teachers felt more confident in their ability to plan mathematical lessons in grade 8 and thus, lesson quality had been improved. In the first implementation of the lesson study, pre-service teachers had a basic performance in delivering the lesson to their students. As the innovation progressed, the pre-service teachers were getting better in delivering the lesson. In the last implementation of the lesson study, they were considered proficient pre-service teachers in terms of instruction. Pre-service teachers started from teaching the collaboratively planned lesson and gradually progressed to showing their own style of teaching. The lesson study clearly afforded the pre-service teachers the opportunity to explore different ways to teach. Furthermore, the pretest mean scores in the self-confidence survey were classified as showing high confidence while the posttest mean scores were classified as showing very high confidence. In addition, the pre-service teachers increased their confidence to teach mathematics from collaborative planning, teaching, debriefing, revising, and re-teaching. Significant increase existed in the self- confidence of pre-service teachers before and after the conduct of the lesson study. Likewise, there was a significant improvement in the mean scores of pre-service teachers' teaching effectiveness between the first and last implementation of the lesson study. Finally, pre-service teachers were able to produce lesson plans as resource materials for mathematics as a result of lesson study.