Aim To observe the effects of G protein inhibitory polypeptide (GCIP)on the proliferation of rat vascular smooth muscle cells (VSMC) promoted by angiotensinⅡ (AngⅡ) in vitro. Methods VSMC were isolated from the media of rat's thoracic aorta obtained by explanted method, and identified by immunocytochemistry assay (α-SM-actin). The experiment was carried out with the AngⅡ to induce cell proliferation, and the GCIP-27 of different density was joined at the same time, then MTT assay, Bradford method and cells count were utilized to test the proliferation and the level of vascular smooth muscle cell protein. Results Compared with the control group, the VSMC proliferation activity, protein level, and cell sum (by number) were obviously increased in AngⅡgroup ( p<0.05 or p<0.01). Meanwhile, GCIP-27 (≥10 μg/L) markedly inhibited the proliferation activity , protein level, and cell sum with a significant difference from AngⅡgroup ( p<0.05 or p<0.01). Conclusions GCIP-27 could inhibit proliferation of VSMC induced by AngⅡ.