The purpose of the study is to examine the communication effects of national leaders on social media, noting that national leaders in each country are actively engaged in public diplomacy using social media today. Through experiments and surveys, the present study examine the influence of social media interactivity and the topics of messages delivered by national leaders on organization-public relations by separating the interactivity into low and high degree, and by separating the topics of messages into soft power topics and hard power topics. The present study also examines the influence of organization-public relations on national reputation. In addition, this study examines both the moderation effects of parasocial interaction between social media interactivity and organization-public relations, and the mediation effects of organization-public relations between social media interactivity and national reputation. This study used experimental and survey research methods, selected German Chancellor Angela Merkel, and manipulate her Twitter into an experimental scenario. Also, the experiment was designed by classifying soft and hard power topics around the issue of 'COVID-19', which is currently the most important and most closely related issue in the world. The present study utilizes a 2 (topics of messages: soft power topics/hard power topics) X 2 (high interactivity/low interactivity) between-subjects factorial design. The main results of this study are as follows. The influence of social media interactivity on organization-public relationships was significant, but the influence of message topics (soft power topics, hard power topics) on organization-public relationships was not significant. Nevertheless, interaction effects on social media interactivity with the organization-public relationship of the message subject was significant. The influence of organization-public relations on national reputation was significant, and parasocial interaction have the moderation effects between social media interactivity and organization-public relations. Also, organization-public relations have partial mediation effect between social media interactivity and national reputation. This study provides not only academic insights into PR research using social media interactivity, soft power, hard power, and parasocial interaction which is the concept of media psychology, but also provides various practical implications for public diplomacy using social media by national leaders.