This study was performed to identify nutritional status and eating behaviors of underweight male adolescents aged 15 to 19 years. The subjects selected by using the data of KNHANS-2001 were 32 underweight boys and 135 normalweight ones. We found that weight, waist and hip circumference of the underweight group were significantly different to normal-wight group, but height and waist-hip ratio were not. Their serum indices belonged within normal ranges and showed no difference between the two groups. Their energy and nutrient intakes were mostly poor. The level and proportion of the subjects below EAR, NAR and INQ of each nutrient were not significantly different between the two groups. There were no difference of frequencies of skipping meals, snacking and eating-out between the two groups, either. When comparing frequencies of food intakes, the underweight group consumed significantly more of fermented fishes and less milk than the normal-weight group. And the former had significantly more rest/sleep, no-does and supplementation and less regular excercise than the latter. The underweight group perceived more correct self-images than the normal-weight group and they tried more to increase their body weight during weight control practice (p<0.001). It was concluded that the underweight group showed no different biochemical indices, nutrient intakes, and dietary behaviors to the normal-weight group, but they revealed significantly higher non-active activities like rest and supplementations.