The consistent poor achievement of students in physics tests and in science generally is one problem attracting researchers because of the danger it poses to the nation's technological advancement. This work focused on the effect of content thought on students' achievement. Two research questions were put forward which led to the formulation of two hypotheses tested at 0.05 level of significance. Two instruments were designed for the study. They include Physics Content Coverage Rating Scale (PCCRS) and Physics Achievement Test (PAT). The reliability of the instruments was established. The study design is a descriptive survey. The sample of the study consists of 10 physics teachers and 482 Senior Secondary II (SSII) physics students from 10 selected schools in Delta North Senatorial District of Delta State, Nigeria. The findings of the study include among others: an increase in content taught leads to improved students' achievement in both Ministry-made test and a general improvement in Teacher-made test over that of Ministry-made Physics tests. The following recommendations among others were proffered: teachers should be encouraged to attend seminars and conferences to improve their knowledge of the contents they are teaching to avoid deliberate neglect in teaching of some content areas. (Contains 4 tables.)