Science education has undergone many reforms since the past to improve the learning and teaching process. Teachers' knowledge and motivation, and teachers' teaching techniques, determine how students understand science concepts. In the journey of taking students to innovations in education, the teacher who provides science education in the classroom has a great responsibility (Research and Development Branch, 2017). It is also necessary to identify the problems faced by students in building science education. The analysis of the examination department's results and evaluation reports shows that compared to other subjects, the minimum mark is obtained for the science subject. The Sri Lanka Examination Department's 2017 G.E.C. (G.E.) evaluation report also shows that among the three fields of science, biology, chemistry, and physics, the facility for the field of chemistry is low. For this reason, it was the main interest of this research to find out the correlation between student performance and conceptual understanding of students who learn the content of the chemistry subject of the science subject. For that purpose, the size of the sample was determined using the table of determination of the sample size presented by Krejcie & Morgan (1970) and the correlational study research strategy using 400 students of 08 secondary schools in the Kegalle district of Sri Lanka selected through a cluster random sampling method as the sample. The research was conducted using a quantitative design. For this, Katerina Salta & Chryssa Tzougraki (2004) used an aptitude test set according to the Liket scale and a structured type of achievement test set according to Bloom's classification to find out the level of concept perception of 11th grade students in a school in Greece. A control test was administered to 10% of the research instrument sample. By analyzing the data collected, the research instruments were standardized. Then by giving the research equipment to the main sample, the collected data was analyzed, and conclusions were drawn from the data interpretation. Here, a positive correlation of Pearson correlation coefficient (r= + 0.720) was found between students' conceptual understanding and student achievement. It was recognized that appropriate student-centered methods should be used in the learning and teaching process as student achievement increases when students' conceptual understanding increases.
Conceptual Perception, Student achievement, Chemistry, G.C.E (O/L) examination.