Aim To investigate the effects of medium chain triglyceride (MCT) on the proliferation of rat aortic smooth muscle cells (RASMC). Methods Cultured cells were used in this experiment. Oil red staining was used to observe changes in cell morphology. Effect of MCT on RASMC proliferation was tested by MTT and flow cytometry. Results Oil red staining results showed that the number of lipid-infiltrated cells increased as the concentration of MCT increased, the morphology of the cells changed more obviously at 2000 mg/dL, the cell body became larger, round and irregular. Effect of MCT on the proliferation of RASMC was in a time- and concentration-dependent manner. Promoting proliferation was observed on 24～48 h and 62.5～250 mg/dL, the highest proliferation rate was 132.9% (t=4.5529, P=0.0005) at 24 h and 62.5 mg/dL; On 24～48 h and ≥250 mg/dL, 24～144 h and ≥500 mg/dL, the inhibitory effects on proliferation was observed in each test group, the lowest proliferation rate was 1.0% (t=68.8652, P<0.0001) at 120 h and 1000 mg/dL. Flow cycle results showed that G₀/G₁-phase percentage of MCT group reduced (F=5.56, P=0.0039) versus the control group, while percentage of S-phase increased (F=7.28, P=0.0011). In addition to 24 h, the results of cell apoptosis test showed a decreasing in apoptosis percentage of MCT group compared with the control group; On 48～144 h, the apoptosis rate was 2.6% (F=1.86, P=0.1591), 0.9% (F=6.43, P=0.002), 0.9% (F=3.41, P=0.0274), 1.0% (F=5.76, P=0.0033) and 2.1% (F=5.18, P=0.0054), respectively. Conclusion Effect of MCT on the proliferation of RASMC is bidirectional, low concentration with short time showed promoting effect, and the moderate and high concentration showed inhibiting effect.