Reverberation reduces speech quality, and therefore causes inconveniency to listeners, especially those using assistive hearing devices. To enhance the quality of reverberant speech, a significant step is speech quality assessment, most of which are based on subjective judgements. Subjective evaluations vary with listeners' perception, emotional and mental states. To obtain an objective assessment of speech quality in reverberation, this work carried out an event related potential (ERP) study using a passive oddball paradigm. Listeners were presented with anechoic speech as standard stimuli mixed with reverberant speech under different levels of reverberation as deviant stimuli. The ERP responses reveal how human-beings' subconsciousness interacts with different levels of reverberation in the perceived speech. Results showed that the peak amplitude of P300 in ERP responses followed the variation of reverberation time in reverberant speech, providing evidence that P300 in ERP responses could work as a neural surrogate of reverberation time in objective speech quality assessment.