Arachidonic acid (ARA) is an n-6 PUFA and is thought to have an important role in various physiological and psychological functions. Recently, supplementation with ARA-enriched TAG was shown to improve age-related decreases in cognitive function in healthy elderly men. To investigate the influence of baseline serum ARA status on cognitive function and its improvement, we analyzed cognitive function stratified by serum ARA level. The stratified analysis was also conducted for the effects of ARA-enriched TAG supplementation on cognitive improvement. Cognitive function was evaluated by measuring event-related potentials (ERPs), including P300 latency and amplitude. When participants were stratified by baseline serum ARA level, P300 latency was significantly longer and P300 amplitude was generally lower in the low-ARA group than in the high-ARA group. No significant difference in P300 components was observed when participants were stratified by serum levels of any other fatty acid. ARA-enriched TAG supplementation significantly shortened P300 latency and increased P300 amplitude in the low-ARA group, although no significant differences were observed in the high-ARA group. These findings suggest that lower serum ARA levels were associated with cognitive function in elderly men and that ARA-enriched TAG supplementation is more effective in improving cognitive function in healthy elderly men with low serum ARA levels than in those with high serum ARA levels.