The low mathematics learning achievement of students motivated this study. As a result, teachers employ demonstrative learning approaches in order for students' learning achievement to improve. The purpose of this study was to determine the effect of the demonstration learning method on mathematics learning achievement on topic flat geometry at fourth-grade of SD Negeri Weton-wetan. This research used quantitative research. The population in this study was fourth-grade students. The sample used in this study amounted to 30 fourth-grade students. This research method used experimental research methods. The instrument used in this study was a grid of pretest and posttest questions for fourth grade SD Negeri Weton-wetan. Pretest and posttest were used in this data collection technique. The average posttest score is greater than the average pretest score, according to the findings of the data test using the SPSS application (86.77 56.10). The data can be stated to be normal because the sig value is higher than the alpha value (0.806 0.05) and the normality test demonstrates that the pretest and posttest data are normal (0.806 0.05). Meanwhile, based on hypothesis testing results, Ho is rejected and Ha is accepted since the value of Sig (2-tailed = 0.000) 1/2 (0.05) indicates that Ho is rejected and Ha is accepted. So, based on these three results, it can be stated that learning by using the demonstration method has a very significant effect on the results of learning mathematics achievement in fourth-grade students, especially flat geometry material, at SD Negeri Weton-wetan.