Effect of docosahexaenoic acid (DHA) [22: 6(n-3)]-fortified Chlorella oil fraction on radial maze performance was studied in aged mice. Male ICR mice aged 9 months were fed a diet containing 2 g DHA-fortified Chlorella oil fraction/100 g diet or normal diet (Control group) for 2 months. Two months after the start of feeding, the mice were tested for learning ability related to 2 types of memory, reference memory and working memory, with the partially (4 of 8) baited eight-arm radial maze. Reference memory is a kind of information that should be retained until the next trial. Working memory is a kind of information that disappears in a short time. Entry into the unbaited arms and repeated entry into the visited arms were defined as reference memory errors and working memory errors, respectively. DHA-fortified Chlorella oil fraction administration to mice for 2 months resulted in a significant decrease in the number of working memory errors without affecting the number of reference memory errors. A significant increase in the DHA content in the brain was also observed. These results suggest that the intake of DHA-fortified Chlorella oil fraction effectively enhances working memory in maze performance.