The study investigated the effect of positive emotion induced by olfactory stimuli (Aromatic) on the memory span under conditions of varying task difficulty. Eighty-two healthy college students performed the easy/difficult task while positive or neutral emotion was induced by aromatic/neutral olfactory stimulation in two separate experiments. Results showed that compared to neutral condition with air stimuli, a significant performance improvement in the presence of orange odor when the memory task was reading working memory span (the difficult task) but not the simple number span (the easy task). Our results provided the empirical demonstration that positive emotion induced by aromatic smell could facilitate memory performance, and also highlighted the potential modulatory role of task-difficulty. This result deepens the broaden-and-build theory of positive emotions.