Due to the unique physical and chemical properties, transition metal carbides (TMCs) have many potential applications in the fields of energy conversion and catalysis. Chemical vapor deposition (CVD) is a promising method to synthesize TMCs. However, spatially inhomogeneous supply of transition metal precursor vapor in the normal CVD process generally leads to poor control of the morphology and uniformity of the products. Here, we report a vapor-liquid-solid (VLS) growth process where non-volatile Na2MoO4 is used to act as a liquid precursor for the growth of uniform ultrathin Mo2C nanosheets on Al2O3(0001). The morphology of the nanosheets can be controlled by tuning the precursor concentration, annealing time and growth temperature. The roles of Na and the liquid-solid interface in consolidating Mo atoms and promoting the epitaxial growth of Mo2C nanosheets are demonstrated. Furthermore, we show that the liquid-solid interface can cause the crystalline phase transition of Mo2C nanosheets through verification experiments.