Electrophysiological techniques were used to examine the effect of word class ambiguity on the brain response to Chinese noun-verb word class ambiguous words, namely, whether noun-verb word class ambiguous words and word class unambiguous items (nouns and verbs) engage different neural mechanism in grammatically well-specified contexts. There were three sets of stimuli used in the experiments: (1) word class unambiguous nouns, (2) word class unambiguous verbs, and (3) noun-verb word class ambiguous words(can be used as both nouns and verbs, but nouns and verbs senses have little or no semantic ambiguity). Words length and word frequency were matched across three word types. These stimuli were embedded in two contrastive grammatical phrase contexts that explicitly specified their word class: “yi ge(一个)+ _”(e. g., noun-predicting) and “bu yuan(不愿) + _” (e. g., verb-predicting,). All the phrases were ranged randomly and presented visually word by word. The subjects were asked to decide whether the presented phrase represented a legal or illegal phrase. ERPs in response to the nouns, verbs and noun-verb ambiguous words in different contexts were recorded. The ERPs were then analyzed using a repeated -measures ANOVA. Fig. 2 showed significant ERP differences between noun-verb ambiguous words and word class unambiguous word in intervals of 270-350ms. Unambiguous words elicited a larger N400 than did noun-verb ambiguous verbs over frontal and central sites, p=0.016. But the N400 effect only appeared in the verb-predicting context(see fig. 4), p=0.01. There no significant P600 effect between noun-verb ambiguous words and word class unambiguous word in intervals of 450-600ms. Fig. 6(left) showed significant word class effect for unambiguous nouns and unambiguous verbs. When the context completely matched their grammatical role, compared to unambiguous nouns, unambiguous verbs elicited a larger N400 over frontal and central sites, p=0.046, and a smaller P600 over posterior areas, p=0.04. Similarly, word class ambiguous words used as nouns elicited a larger P600 than did word class ambiguous words used as verbs(see Fig. 6 right), p=0.05. P600 was the index of grammatical process which elicited by grammatical features of word class in our experiment. Thus, according to the present data, we argue that noun-verb word class ambiguous words in Chinese are not special neural mechanism compared to word class unambiguous nouns and verbs.