This study aims at investigating the effect of teaching Science using the scientific activities methodology on the achievement of eighth grade students who have different mental capacities of the scientific concepts and the scientific attitudes development. The sample of the study is (65) male students from eighth grade distributed randomly into two groups: experimental and control. According to their results in the mental capacities exam, the students of each group are distributed into two sections: high mental capacity and low mental capacity. The study tools were: mental capacity exam, the scientific concepts exam, and the scientific attitudes scale. The analysis of the results of the two groups used is the 2-way MANOVA . The results show that there is a statistically significant difference at (α = 0.05) between the two means of the scores for the experimental and control groups in the scientific concepts exam and the scientific attitudes scale due to the teaching methodology in favor of the experimental group which used the scientific activities methodology. The results also show that there is a statistically significant difference at (α = 0.05) between the two means of the scores for students with high mental capacity and the scores of those with low mental capacity due to the level of the mental capacity in favor of students with high mental capacity. There is no statistically significant difference spotted at (α = 0.05) between the two means of the scores for the experimental and control groups due to interaction between the teaching methodology and the mental capacity level. In the light of the results of the study, the researcher recommends the following: Science Teachers’ Training , before and during service, on using scientific activities methodology, is a necessity for teachers of Science to use and activate the scientific activities methodology in the classrooms and labs, let alone science curricula should focus on scientific activities. It is also recommended to conduct other researches and studies to investigate the effect of the scientific activities methodology using other variables other than those mentioned in this study, such as innovative and creative thinking.