Writing requires students’ ability to analyze and memorize. Therefore, having an adequate learning medium such as the Learn English British Council website was very helpful in learning activities, because it offers tips and exercises for writing skills. This research aimed to find out the effect of the Learn English British Council website on students' writing skills in the 7th grade of SMP Negeri 1 Purwokerto. This research used quantitative with a quasi-experimental design. The study's population consists of 96 students. The samples consists of 64 students and were taken by simple random sampling. The experimental is 7-C and the control class is 7-A. The data collection used pre-test and post-test. The data was analyzed by IBM SPSS Statistical 25 by using the Mann-Whitney U-test. The results showed significant differences in students' post-test scores between experimental and control classes, with a sig (2-tailed) value of 0.000, rejecting the null hypothesis (H0) and accepting the alternative hypothesis (Ha). Then, the researcher measured the differences in students' post-test scores between experimental and control classes by using Cohen’s standard effect size formulation with a result of 1.9. It showed that using the Learn English British Council website had a significant effect on students’ writing skills.