To observe the therapeutic effect of Kouchuangping granule (KCP) on recurrent aphthae and explore its mechanism. One hundred and forty-eight patients treated with KCP were observed and compared with 49 patients treated with tripterygii multiglycoside tablet as control in controlling relapse, prolonging intermittent period, improving local symptom and the influence on humoral and cellular immunity. In animal experiment, the effects of KCP on immunologic function, anti-exudation, anti-inflammation and analgesia were studied. The clinical markedly effective and cure rate of the KCP group was 62.8% and the total effective rate was 89.9%, which were significantly higher than those of the control group (40.8% and 75.5%, P < 0.01, P < 0.05). In the KCP group, the intermittent period of aphthous ulcer recurrence was prolonged to 6 months and the local symptoms were markedly improved, the differences were significant as compared with those before treatment and those in the control group (P < 0.01 or 0.05). The KCP showed effects in suppressing humoral immunity and enhancing cellular immunity. Results of experimental study showed that KCP could inhibit the supernormal reaction of cellular immunity to prevent and treat the hypersensitivity caused erosion and ulceration of mucosae, inhibit capillary permeability and WBC migration and abate the 5-hydroxytryptamine (5-HT) and histamine induced local swelling and pain. KCP has a significant effect in treating recurrent aphthae, the mechanism might be related with its effect on immunoregulation, anti-inflammation and anti-exudation.