This study compared the relationship between the use of Persian vs. English in teaching English grammar to Iranian students and their achievement in learning English grammar. The participants of this study include 50 female students from a junior high school in Mehran. The researcher randomly selected 2 groups, one group was taught through the medium of Persian, while the other group was taught the same material through the medium of English. The materials were based on the content of the 3 lessons of third grade English text-book. Then a 30-item achievement test (pre-test) was administered to two groups to be getting insured of their homogeneity. The instruction began in two groups separately and the treatment lasted for 5 weeks. After the end of the instruction, the test was administered to two groups as post-test again. The data analysis showed that two languages (Persian & English) lead to positive learning outcomes in the post-test, although the instruction through Persian caused better results than the instruction through the English.