The present invention relates to a composition for preventing or treating Behcet’s disease and rheumatoid arthritis, the composition containing a peptide or a mixture thereof as an active ingredient. More specifically, an intraperitoneal injection or application of P1 to P5 peptides or a mixture thereof as a skin external preparation to a mouse animal model having Behcet’s disease caused by autoimmune response was found to ameliorate skin ulcers and skin inflammation in the Behcet’s disease mouse model, and an arthritis amelioration effect on a rheumatoid arthritis animal model was also observed. Accordingly, the composition containing P1 to P5 peptides or mixtures thereof as active ingredients can be provided as medicine for Behcet’s disease and rheumatoid arthritis.