Abstract: Objective To investigate the expression of miR-21 and its target protein RASA1 in skin of patients with psoriasis vulgaris and its significance in the development of psoriasis vulgaris. Methods The expressions of miR-21 were studied with real-time fluorescent quantitative PCR in the 12 psoriatic vulgaris lesions and beside-lesional tissue and 15 normal skin, differential multiple representation with 2^-ΔΔCt.The expressions of RASA1 were studied with immunohistochemical in the 45 psoriatic and 20 normal skin.The positive mainly as brown or tan particles in cytoplasm. Results With real-time fluorescent quantitative PCR, the expression level of miR-21 was higher in lesions of psoriasis vulgaris than that in the beside-lesional tissue and normal skin tissue, and differences was statistical significance (P < 0.01).The positive expression of RASA1 in skin lesions of patients with psoriasis population was 10.00%, it's lower than 60.00% the rate of positive expression in the normal group.The data were statistically significant (χ²=14.009, P < 0.001). Conclusion The expression of miR-21 is higher in lesions of psoriasis vulgaris than that in the normal skin tissue, but the positive expression of its target protein RASA1 is lower in lesions of psoriasis vulgaris.Differential expression of miR-21 may be regulation of RASA1 gene, which can inhibit the expression of RASA1 protein.These may lead to psoriasis vulgaris.