The aim of this study is to investigate the effect of using activities based on the Montessori Approach in the academic achievement and science Process Skills of the fourth grade. A qusai -experimental design was used where the study divided into two groups. The experimental group consisted of (31) students and it was selected from the fourth grade in schools of Firq Basic Education (1-4) whereas the control group consisted of (31) from and it was selected from Gwaher AL-Adab (1-10) school in Nizwa. The study lasted 8 weeks during the second semester 2016/2017 To achieve the aims of the study. Montessori Hall has been set up with six dimensions and each comer has several shelves containing educational activities. In addition, teacher guide was prepared to guide the teacher to implement the Montessori Approach for the experimental group. Two instruments were designed to answer the two research questions. These are achievement test consisted of 13 items and science processes skills consisted of 20 items. The validity of both was checked through panel of judges and the reliability was checked by internal consistency methods using Alfa Cronbach. The value of Alfa coefficient for the achievement test is (0.91).whereas for the science reprocess skills is (0.82), which makes those methods suitable for the study. The results of the study indicated that there were statistically significant differences at the level of significance (a= 0.05) between the mean scores of the experimental and the control groups in the academic achievement in favor of experimental group. In addition, the results showed statistical significance differences at the level of (a = 0.05) between the mean scores of two groups in the science processes skills in favor of experimental group. In the light of the previous results, the study recommends holding training workshops for teachers in the field of science teaching using the Montessori's approach and to train them on how to develop science processes skills among fourth grade students. Finally, conducting further studies using Montessori's Approach with other variables like critical and creative thinking as well as problem solving skills.