This paper aims to explore topic effect on syntactic complexity in Chinese EFL learners' and English native speakers' writings. To this end, the paper examined 600 argumentative essays on two topics, using 14 measures of syntactic complexity to investigate whether there is significant difference in topic effect on EFL writings and ENS writings. Four dimensions are included in these 14 syntactic complexity measures, which are "length of production unit," "amount of subordination," "amount of coordination and degree of phrasal sophistication." Results showed strong topic effect on both Chinese EFL writers and ENS writers in terms of "length of production unit" and "particular structure." The topic effect differs in "amount of coordination" in that the Chinese EFL group produced different amount of coordination in the two topics, while the ENS group did not show significant difference in this aspect.