Numerous studies have demonstrated that word frequency influences sentence processing. However, the impact of frequently used epistemic modality adverbs on sentence processing is less clear. The aim of the present study is to investigate the processing of Chinese epistemic modality adverbs by means of ERPs (event-related potentials) based on degrees of modal certainty encoded by different orthographic forms of two epistemic adverbs yexu (possibly) and yiding (surely) co-occurred with two psychological verbs guji (estimate) and quexin (convince) in Mandarin Chinese sentences as Wo guji Xiaolin yexu hui jingxuan banzhang (I estimate Xiaolin possibly future compete monitor). Two conditions (i.e. agreed and disagreed conditions) are constructed through manipulating the agreement of the certainty degree between the psychological verbs and the epistemic adverbs. Twenty-four enrolled Chinese college students took part in the ERP experiment. The results showed that the epistemic adverb yexu (possibly) in disagreed sentences (quexin &yexu) relative to agreed sentences (guji &yexu), elicited a monophasic P600 effect with a centro-parietal distribution, indicating extra syntactic costs in processing the epistemic adverb in sentences with modal uncertainty. The absence of N400 was observed for processing yexu (possibly) in the agreed and disagreed sentences, suggesting that little semantic processing difficulty of the epistemic adverb happens in sentences with modal certainty and/or uncertainty. While for the epistemic adverb yiding (surely) in the agreed and disagreed sentences, the absence of P600 and N400 indicates that little difficulty in syntactic and/or semantic processing of the epistemic adverb happens in sentences with modal certainty and/or uncertainty.