This article reports a study on the impact of L2 Korean on L1 Chinese lexical diversity and grammar in written expressions by Chinese bilinguals proficient in Korean. The statistical analysis showed that the cross-linguistic effects of L2 on L1 were significant although such impact was bidirectional. There were significantly more grammar errors and longer retrieval time committed by the bilingual group which implied negative L2 transfer to L1. Meanwhile, L2 also showed a positive influence on lexical diversity as there was no decline in lexical richness but an increase in lexical variations, and this indicated that L2-induced patterns did not replace or deteriorate L1 but instead added additional options to L1 expressions. This phenomenon can be characterized as the addition of new concepts and linguistic options to the already-existing L1 and conceptual repertoire. Specifically, meta-linguistic competence was enhanced. This research supports the theory of interference on L1 by the use of L2 (negative transfer), but it could also be enhanced by L2 (positive transfer).