This study examined various measure units to analyze syntactic complexity shown in Korean learners` writings, and to figure out indexes to predict the proficiency. Complexity measured by conjunction and coordination showed no significant relation with proficiency; however, subordination showed significant relation with clauses, T-units, and sentences. Complexity measured by sentences showed higher correlation with the proficiency, verifying that sentences could substitute the role of T-units in measuring complexity of Korean writings. The measures by clause and subordinate clause showed significant correlation with every skill of proficiency. This study verified that the number of subordinate clauses per clauses, subordinate clauses per sentences, and clauses per sentences were the three most important indexes to measure complexity in L2 Korean writings.