The present study aimed to investigate the effect of using chatGPT on developing the third preparatory Egyptian students' performance in English grammar rules. The English grammar rules pre-posttests were designed in line with the participants' third preparatory curriculum for the purpose of data collection. Two classes of sixty students were selected for the purpose of conducting the current study. The two classes were equally assigned as a control group and an experimental group. Being an experimental research, the current study employed the mixed method of both types of analytical approach: quantitative and qualitative. The former type includes the use of descriptive statistics of the mean scores and standard deviations of both groups' pre-and posttest performance. In addition, the former type employed the paired samples t-test and the independent samples t-test. The latter type covers the description of both groups' per-and-posttest performance. The results revealed that the experiment group's English grammar was enhanced due to the use of chatGPT. Pedagogical implications and recommendations were provided.