This study examines the effect of familiarity, context, and linguistic convention on idiom comprehension in Mandarin speaking children. Two experiments (a comprehension task followed by a comprehension task coupled with a metapragmatic task) were administered to test participants in three age groups (6 and 9-year-olds, and an adult control group). Laval (Journal of Pragmatics 35(2):723-739, 2003) showed that familiarity had an effect on idiom comprehension for French 9-year-olds. However, our finding showed that familiarity was important for 6-year-old Chinese children when a context was not given. Abkarian et al.(Journal of Speech and Hearing Research 35:580-587, 1992) claimed that context has little or no effect on comprehension for children under 6. Our results show that context has an effect on 6-year-old children's understanding of idioms in a different way. Overall, our major research findings are: (1) Familiarity first appeared in responses at age 6. (2) Context played an important role in idiom comprehension and had different effects on different age groups. (3) Linguistic convention starts from age 6 on, and a significant effect took place at the age of 9. (4) Metapragmatic knowledge showed at the age of 6 and could surface even younger. As context and linguistic convention have a substantial effect on the comprehension of idioms, it is necessary to take them into account to explain language functioning and communicative situations. (Contains 12 tables, 2 figures, and 2 footnotes.)