This study aims to explicate the structure of humor texts`` and a principles underlying its laugh-generation. This paper defined humor as words and related acts that cause others to laugh, and delineates the domain of the subordinate concepts. Humor texts were defined as stories with specific structures designed intentionally to elicit laughs from other people. Depending on the format, humor texts were classified as narrative humor texts and questions and answers humor texts. Generally, the structure of humor texts consisted of a set-up and a punch line. As for the setup, the ending of story was unpredictable or predictable depending on the author``s compositional strategy. In stories with predictable endings where authors provided clues, authors`` intention to produce unpredictability was most effectively reflecte d. Next, principles underlying humor texts`` laugh-generation were examined. The fundamental principle of humor texts`` laugh generation lies in their unpredictability. The unpredictability implies that the receivers are completely incapable of predicting the results the author has intended. The author induces the receivers`` prediction in a certain direction through the flow of the story. Trapped by the author``s scheme, the receivers then predict the outcomes incorrectly. Thus, the effect of unpredictability is produced. As the receivers put incongruent pieces of the story together, laughs are generated. In order to produce the unpredictability, a variety of devises were put in use. These devises contributed to the author``s plan to produce unpredictability.