Deductive reasoning is an inference-making process that allows individuals to draw valid conclusions from a given set of premises. Conditional reasoning is an important form of deductive reasoning which involves making an inference with a given major premise and one of four possible minor premises. People have been shown to be prone to making logical errors in deductive reasoning and to be strongly influenced by the semantic content of logical arguments. This phenomenon is the famous ‘belief-bias effect’. That is, people tend to accept believable conclusions and reject implausible ones, regardless of logicality. Therefore, beliefs that are consistent with the logical conclusion facilitate one’s performance during the logical task; in contrast, beliefs that disagree with the logical conclusion are often inhibitory to the logical task. Take the conditional premise, “If someone is female, then raise your hand”, for example. The belief regarding the content will discourage the logical conclusion under the form (if a person is not female, then it is uncertain whether or not the person should raise his hand). However, in the conditional premise, “If a person is rich, then the person will be happy”, the logical conclusion seems to be ‘‘facilitated’’ in the form (if a person is not rich, then the conclusion is uncertain). Based on the inference-verification paradigm in which the major premise, minor premise and conclusion appear one after another, high-density event-related potentials (ERPs) were recorded to examine the electrophysiologic correlates of belief bias effect in realistically conditional reasoning in daily life. That is, the present study investigated spatiotemporal patterns of the brain in the performance of one conditional reasoning task (“If P then Q, not-P) with two conditions (facilitatory of belief condition and inhibitory of belief condition) using high-density (64 channels) ERP recording system. We aimed to clarify the temporal features of the belief-bias effect and determine ERP components that embody this bias. The ERP waveforms were time-locked to the onset of the major premises, the minor premises, and the conclusion, respectively. The averaged epoch for the major premises was 3200ms, including a 200ms pre-stimuli baseline. The averaged epoch for the minor premises was 3200ms, including a 200ms pre-stimuli baseline. The averaged epoch for the conclusion was 1200ms, including a 200ms pre-stimuli baseline. Only trials with correct judgment were taken into the ERP analysis. Results showed that, first, relative to the facilitatory of belief condition, the behavioral data reflected longer RTs and lower accuracy rates in the inhibitory of belief condition, verifying the belief-bias effect. Second, the components elicited by facilitatory of belief condition and inhibitory of belief condition were not significantly different neither the stages of inference of minor premise nor the stage of judgment of conclusion. However, the components elicited by the two conditions were significantly different during the stage of the semantic representation of major premise, most likely reflects the effect of belief-bias in conditional reasoning.