Writing a self-introduction letter plays a crucial role in showcasing oneself for admission. However, existing studies on self-introduction letters have not adequately revealed how much information should be included for maximum persuasive effect. In this situation, we aim to apply a discussion on the aspect of message sidedness to analyze a more persuasive approach to crafting self-introduction letters. This study specifically focuses on the analysis of two-sided messages within the context of self-introduction letters, considering the nature of these letters that often require the disclosure of applicants' weaknesses. It is based on the practicality of analyzing two-sided messages containing negative information, given the characteristics of self-introduction letters. To examine the effects and interaction effects of the ratio and importance of negative information on factors such as source credibility, attitudes toward the source, attitudes toward the self-introduction letter, and intention of admission, one-way and two-way analysis of variance (ANOVA) were conducted. The results of the study indicate that the ratio of negative information significantly influences attitudes toward the source, attitudes toward the self-introduction letter, and intention of admission. The finding that persuasive effects decline when the ratio of negative information exceeds 40% partially confirms the results of previous research. Meanwhile, the importance of negative information did not exhibit a significant impact. The interaction between the ratio and importance of negative information significantly altered source credibility and admission intention. When the importance of negative information was high, the ratio of 20% resulted in the highest persuasive effect, while when the importance was low, a ratio of 40% led to the highest persuasive effect. This variation in the ratio to be included, depending on the importance of negative information, supports Eisend's (2006) argument that importance should be considered when analyzing the effects of the ratio of negative information. This study not only provides practical insights into how to effectively present oneself in a self-introduction letter for admission but also holds theoretical significance by analyzing the interaction effects between moderating variables affecting message framing, an aspect that has not been extensively explored in previous research. Furthermore, the meaningful changes in source credibility through the interaction of these two variables contribute to the advancement of theoretical discussions on message sidedness.