ABSTRACT This study investigates the factor affecting students’ performance in physics: a case of grade – ten students in Kelela general secondary and preparatory school. The study was conducted using a descriptive survey method. The subjects of the study were 112 students (62 male and 50 female) of grade 10 that represented 13.25% of the total grade 10 students in the study. Furthermore, the samples included in this study are 10 (9 male, 1 female) physics teachers, 1 physics head teacher and 1 school principal. The data gathering instruments that the researcher used were the questionnaire, an observational checklist, interview, and student achievement test. Quantitative data was collected using the questionnaire and student achievement test and analyzed using the table, percentage, frequency, and chart, while qualitative data collected was made using an observational checklist and interview. The study revealed that the primary school performances of most students are poor and their attitude towards the subject is negative. The study also found out all teachers has an academic qualification for secondary school, but most of the teachers can't use different valuable teaching methodology. The teaching-learning resource like plasma, projector, simulation, measuring instrument etc. in the selected school are insufficient. On the other hand, the functionality of practical work/laboratory in the school is very low. Therefore, the factors that are affecting students' performance in physics at Kelela general secondary and preparatory school are students’ poor primary school performance, students’ negative attitude towards physics, unused of different valuable instructional methodology, insufficient of teaching-learning resource and lack of practical work/laboratory.