In South Korea, ants are predicted to shift their distributions northwards and upwards. It was predicted that ant fauna will be changed dramatically in highlands due to the range shifts. The Mymica ants which are most abundant in high altitudinal areas in South Korea will be nearly disappeared there in 2050s, and replaced by Aphaenogaster japonica which is abundant in lowlands. It was recently found that A. japonica shifted upwards in Mt. Hanla in Jeju island, South Korea. Interestingly, A. japonica is similar in shape and size with the Myrmica species, which indicate niche overlap and resulting intensive competition. To find elevational change of competitions between two specie, we investigated ants during two ant foraging periods (2010 and 2011) in a high mountain (Mt. Gaebang) using pitfall traps and bait traps along elevational gradients. Ten ant species were collected from a.s.l. 800 m to a.s.l. 1577 m. Myrmica sp. 1 was most abundant (35% of total ants) and collected at all elevations. Myrmica sp. 2 (19.7%) occurred from 1000 m to 1577 m. Meanwhile, A. japonica occurred up to 1200 m. Fights between ants were observed 22 times; fights between these two species were most frequently found. Although, competitive hierarchy was not recognized due to a low frequency of interspecific encounters, A. japonica is likely to be superior over Myrmica species in food competition when considering the slower and more timid behavior of Myrmica species compared with A. japonica. Therefore, it is likely that A. japonica would replace easily Myrmica species in Korean highlands when thermal barrier (i.e., cold climate in high elevations) will be removed due to climate warming.