This paper investigated a number of common ungrammatical patterns that were found in Chinese EFL leaners' free writings, in order to find useful pedagogical implications for English grammar teaching in EFL setting, especially in China. The corpus of writing data is examined by the author together with a native English teacher. Our findings suggest that misuse of determiners is the most frequent grammatical error in Chinese students' writing. Other ungrammatical patterns such as Chinese-English expression, tense error, misuse of prepositions, are also considerably important and should be given due attention by EFL teachers, especially Chinese EFL teachers in their teaching practice.