This study examined behavioral changes in 15-day learning of word-picture (WP) and word-sound (WS) associations, using meaningless stimuli. Subjects performed a learning task and two recognition tasks under the WP and WS conditions every day for 15 days. Two main findings emerged from this study. First, behavioral data of recognition accuracy and response time were improved in both conditions of WP and WS by the 15-day learning of associations. Second, the performance in WP was better than that in WS for 15 days, but the correlation between accuracies in WP and WS was significant across subjects. These results suggest that even if meaningless stimuli are employed as an experimental stimulus, the long-term learning of associations between words and perceptual features could be formed in both visual and auditory modalities, between which the performance levels are highly correlated, but the learning effect could be dominant when words are associated with visual features, compared to auditory features. (Contains 5 figures.)