On the internet, the enterprise provides service guarantee, such as "return without reason in seven days", to reduce the perceived risk of online customers effectively. Meanwhile, such service guarantee leads some customer opportunistic behavior. Taking the customers' personal characteristics and reference group’s relationship strength as moderator variables, we conduct an empirical research to study the major factor and it’s effect paths on customer opportunistic behavior by using the scenario role-playing approach. The result shows that higher service guarantee is more likely to induce customer opportunism behavior. And customers’ personality (Machiavellianism) has nothing to do with the relationship. On the contrary, the relationship strength has a significantly moderating role in the impact of service guarantee strength on customers’ opportunistic behavior. Knowing friends of strong relationship have opportunistic behaviors, customer is more likely to choose the similar behavior when they face the higher service guarantee.