In order to maintain positive social interactions, uninvolved third-parties tend to punish free riders who contribute less than their fair share in a public-goods dilemma. Previous research showed that the quantity and source of endowments can modulate behavior in this situation. The present study combined behavior with Event-related Potentials to examine how the quantity and source of endowments modulate associated brain activity of third-party evaluation of the member contributions and the punishment choices. Twenty-six participants participated in the experiment. We used a within-subjects design with Endowment Source (effort-based or randomly assigned), Endowment Quantity (low or high), and Contribution (low, equal, or high) as factors. Participants joined the study as third-party punishers and were presented with information about other participants' performance from a classical public-goods game (PGG), including the quantity and source of endowment and their contribution. Participants were instructed to decide whether and how much they would like to cost their own money to punish a given member. Furthermore, the event-related potentials were recorded while participants were making their evaluations. Behavioral results showed: (1) Participants punish those members with high endowments stronger than to those with low endowments (p<.001); (2) This effect was greater when endowment amount was randomly assigned than when it was gained through effort (p<.05); (3) The effect was greater when the member's contribution was low than when it was high (p<.001). ERP results showed that the quantity and source of endowment modulated ERP components: Feedback-related negativity (FRN) and P3 that is typically associated with the evaluation process. When the initial endowment was high (i.e., 100 RMB), FRN magnitude was more negative when the endowment was assigned randomly than when it was gained with efforts (p<.05). Whereas when the initial endowment was low (i.e.,10 RMB), this difference was not significant. P3 magnitude was smaller when the quantity of initial endowment was high (p<.05). Besides, we found that the difference in the amplitude of FRN between the two sources (i.e., randomly assigned vs. effort-based) was significantly correlated with the difference in punishment strength (p<.05). These results suggested that the neural reactivity (i.e. the FRN) and the overt behavioral index (i.e. the punishment) were closely connected. The present study offer insights into how social norms held by third parties can modulate punishment given to the free-riders in a PGG. The present findings prove that the punishment meted out by third parties in a PGG is influenced by the noblesse oblige effect, that is, the third-party will evaluate the contributions of members according to the quantity and source of their endowments. When the member's contribution does not meet the third-party’s expectations, the third-party would experience unpleasant feelings, as reflected in the FRN at the evaluation stage, and these feelings further drive subsequent punishment. This finding helps understand why people expect high-income, high-social status individuals to make more contributions and when their contributions do not meet the public’s expectations, there will show strong negative emotions towards it.