Project-Based Learning has been the leading strategy used by most of the top educational systems in the world. Authentic learning that addresses the 21st century skills is what PBL offers. However, little research has been done to explore its potential in improving the quality of education in the country and what framework to be used to be able to "curricularize" PBL. The aim of this research is to examine the effect of Project-Based Learning using Understanding by Design framework in improving the academic achievement and attitudes of grade 6 students in science 6. Two sections in the grade 6 level from Sta Quiteria Elementary School in Caloocan City was selected for the study. Group A (Gold) was taught through project-based learning technique and Group B (Garnet) was taught through a more traditional teaching technique. A pretest and posttest was administered on both groups to find out if there is a statistical difference between their achievements. There was a statistical difference between the mean academic achievement scores of pretest and posttest after the intervention. The statistical difference has proven the effectiveness of PBL as a more effective method in teaching science. In addition, motivation and attitude were positively impacted. Further studies and in-service trainings for teachers were recommended to discover the effectiveness of PBL in other subject areas.