Most previous studies found that nouns were acquired predominantly by infants in many languages, thus noun bias was considered to be universal in children's early word learning. On the contrary, verbs were more difficult for children to learn, as they were less concrete and more relational than nouns. However, whether Mandarin Chinese is a verb-friendly language for children or not is still in hot debate. Evidences from natural observations and laboratory investigations were conflict. Researches that used vocabulary checklist found that Mandarin-speaking infants learnt many more verbs than their English-speaking peers did (Tardif, 1996). In the meanwhile, some experimental study found that Chinese children met more difficulties in verb learning tasks than English and Japanese children even in preschool years (Imai, et al., 2008). To further explore the noun/verb bias problem in Mandarin-speaking infants, it is proposed that the ratio of nouns and verbs should be carefully examined in the present study. As it is possible that the ratio of nouns and verbs might vary with children's age. In this study, Mandarin-speaking infants' expressive vocabulary size was examined to explore the ratio of nouns and verbs in a 6-month longitudinal study. In the first investigation, a sample of 110 infants were divided into three age groups, 18-, 24- and 30- month olds. All infants were tested using PCDI (Chinese Communicative Development Inventory - Putonghua Version) and a self-designed questionnaire for parents. The 18- and 24- month olds were re-tested after 6 months, using PCDI. In the present study, the Noun Bias Effect was defined as "(noun vocabulary size - verb vocabulary size)/whole vocabulary size", the result of this calculation was labeled as k. The results indicated that the noun bias effect was small in 18 month old group (k = .07), but it soon enlarged in 24- and 30- month olds (k = .11~.19). In the individual level, we defined the Noun Bias Infants as they learned more nouns than verbs. The results showed that the noun bias infants and the verb bias infants respectively constituted 50% of the whole sample in 18 month old group (p > .05). However, with the increase of age, most (81~100%) of the infants in 24- and 30- month old groups showed noun bias. In summary, the results in the present study indicated that the noun bias effect increased with age in Mandarin-speaking infants. Verb bias infants were more likely to be found in infants who were less than 18 month old, and noun bias infants were more predominant in 24 month old or older children. It was suggested that previous divergence about noun/verb bias in Mandarin-speaking infants might result from the change of noun bias itself. Future studies may focus more on how and why noun bias changes with age in early word learners.