In this study, relationship between friendship problems of high school students and four requirements of assertiveness was examined. Two hundred fifty-four high school students responded to a questionnaire. Results of Quantification Method Type I indicated that the second requirement, control of emotion, and the fourth, self-direction, had a negative linear relationship with friendship problems; as the scores for the requirements increased, friendship problems decreased. When the first requirement, candid expression, was extremely high, the person had problems of fearing being disliked by friends and being inconsiderate. And if the third, consideration for others, was extremely high, the tendency to fear conflicts and being ignored also became high.