Social networks, both online and offline, have a profound impact on people's daily life. We measured online (WeChat) and offline social network tie strength and investigated their effects on social attitudes. In addition, the moderating role of epistemic authority in the relationship between tie strength and social attitudes was examined. In a pilot study with 163 people, two tie strength scales (for online and offline social networks, respectively) both including four sub-dimensions were formed. In addition, a proper target social issue (delayed retirement) was selected for the formal study. In the formal study, 33 participants and their 264 'friends' from 8 groups participated and their attitudes towards delayed retirement policy were measured. We employed a 2 (offline tie strength: strong vs. weak) x 2 (online tie strength: strong vs. weak) x 2 (epistemic authority: high vs. low) design. Attitude agreement scores defined as the attitude discrepancy between the core person and their 8 types of 'friends' were dependent variables. Results showed that the main effect of offline tie strength and epistemic authority were significant, as well as the interaction between offline tie strength and epistemic authority. That is, the stronger the offline tie strength, the higher the attitude agreement between core persons and their 'friends'. And this effect was more significant when epistemic authority was low as compared to high. On the other hand, online tie strength had no direct impact on attitude agreement. However, it might indirectly enhance the attitude agreement in offline social networks. The implications of online and offline social networks for social attitude change are discussed.