This study examines the nature of stop accuracy and substitute patterns of word-initial Japanese and Mandarin stops produced by Mandarin-Japanese bilingual children. The purpose of the study is to understand phonological development in bilinguals. The sample consists of 36 bilingual children between the ages of three and six, who simultaneously acquired Japanese and Mandarin from birth. The results were as follows: (1) most of the bilingual children were able to produce Mandarin and Japanese stops by the age of three and the accuracy of the target stops were found to develop with age; (2) the age of developing the target consonants is slightly different in the two languages; (3) substitution patterns observed in each language reveals a mixture of child-specific patterns, language specific systems and language influence as well as individual differences. These findings indicate that Mandarin-Japanese bilingual children possess a unique phonological development system, which is a monolinguallike pattern with cross-linguistic interaction. These results constitute a new body of descriptive reference materials documenting the phonological development of bilingual children for speech therapists or pathologists.