The purpose of this study was to examine the effect of gratitude-expression skills on the benefactor in a dilemma situation. The method experimentally employed a confederate who met participants for the first time and executed gratitude-expression skills on them in a “Prisoner’s Dilemma Game” (PDG). An experimental group in which the confederate executed the gratitude-expression skills was compared with a control group in which the confederate did not do so. The results showed the experimental group had higher average and medium or higher effect size regarding the participants’ cooperative behavior in a PDG, the state of reciprocity consciousness, and interpersonal attraction for the confederate than the control group. This study found the positive effect of gratitude-expression skills on motivating the benefactor’s cooperative behavior and enhancing reciprocity consciousness and interpersonal attraction for the confederate even in a dilemma situation.