Aim To study the relationship between the phenotypic differentiation of adventitial fibroblasts and the early atherosclerotic lesion formation. Methods ApoE^-/- mice and wild-type C57BL/6 black mice of 6 weeks were used in this experiment. All animals were fed hyperlipidic diet for 2, 4 or 8 weeks. The ascending aorta was removed for serial sectioning. Some sections were stained with immunohistochemistry to observe the expression of α-smooth muscle-actin（α-SM-actin） and transforming growth factor beta 1 （TGF-β₁ ） in the adventitia at different time points. In addition the arterial adventitial fibroblasts derived from ApoE^-/- mice and C57BL/6 mice after hyperlipidic diet for 2 weeks were cultured. The expression of α-SM-actin was examined by immunofluorescence staining and Western Blot. The ultrastructural changes were observed by transmission electromicroscope. The expression of TGF-β₁ protein was verified by Western Blot. Results In vivo the adventitia exhibited positive expression of α-SM- actin after hyperlipidic diet for 2 and 4 weeks but weakly positive staining for α-SM-actin expression after hyperlipidic diet for 8 weeks. TGF-β₁ expression increased gradually in the adventitia with increasing duration of the hyperlipidic diet. However there was no α-SM-actin and TGF-β₁ expression in the adventitia of C57BL/6 mice all the time. In vitro part cells of ApoE^-/- mice showed positive α-SM-actin and obvious increase of filaments. But that was not found in C57BL/6 mice. The protein levels of α-SM-actin and TGF-β₁ were higher than C57BL/6 mice（P<0.05）. Conclusion The adventitial fibroblasts were differentiated into myofibroblasts at the early stage of atherosclerosis.