This study examines how complex linguistic structures are acquired in child English as a second language. The spontaneous speech producing by a Japanese primary school child, learning English in a naturalistic environment, was audio-recorded regularly over two years and the development of complex syntactic structures containing subordinate clauses was compared with the acquisition of other English morphosyntactic structures as represented within Processability Theory (PT) (Pienemann, 1998; Pienemann, Di Biase, & Kawaguchi, 2005). Although PT predicts that subordination is acquired at the highest stage in processability hierarchy, the results in this longitudinal study show that some of the subordinate constructions emerge at earlier stages in child ESL acquisition.