The main purpose of this study was to examine the effectiveness of guided inquiry learning method through 5E instructional lesson design in conceptual understanding of students on the concepts of atmospheric pressure. The study has three basic research questions. This study was a quasi-experimental design. The study was done with grade 8 ―A‖& ―C‖ students of Haromsaprimaryschool in special Oromo Zone of Amhara Regional State. The sample was selected using simple random sampling for selected the school and purposive techniques also used to selected the classes depend on their previous semester results; the sections were selected for experimental group (n=28) and control group (n=25). The experimental group used guided inquiry learning with 5E learning model lesson design and the control group used conventional teaching method. The data were analyzed through quantitative (paired t-test and independent t-test) and qualitative method. The study used tests and observation as the instruments for collecting the data. The finding of the study were the students who learnt with guided inquiry learning with 5E learning model lesson design was increased their achievement, participation and reflection of conceptual understanding on atmospheric pressure than those who learnt by convectional teaching method. It was concluded that guided inquiry learning method more effective than that of convection teaching method to achieve the students‘ conceptual understanding on the concept atmospheric pressure and further study need to be conducted for identifying why students have lack of conceptual understanding. Lastly, the researcher would like to recommend that teacher who teaches those students to implement guided inquiry learning with 5Einstructional learning lesson design to teaching physics at primary school.