This study investigated the influence of grade level on the development of science epistemological beliefs and the relationship between science epistemological beliefs and science achievement among co-educational secondary schools of Homa Bay County, Kenya. The study employed cross-sectional and correlational survey designs with purposive sampling. Epistemological Beliefs Questionnaire (EBQ) was used to measure science epistemological beliefs. The instrument was administered to 214 students from 2 co-educational schools (Grade 9, n = 116, Grade 12, n = 98). Students' achievement in Biology, Chemistry and Physics were computed for Science Achievement Scores (SAS). The data were analysed by grade level using independent sample t-tests and by dimensions and achievement scores using multiple regression analysis. The findings indicate statistically significant grade level differences in terms of source, certainty and development and non-significant grade level differences in terms of justification. The findings also indicate that certainty and justification dimensions were significant predictors of science achievement. It is concluded that grade level has an influence on development of epistemological beliefs (source, certainty and development) and certainty and justification dimensions were predictors of science achievement. Implications for practice and further research are herein explained.