We fabricate MgB2 ultra-thin films via hybrid physical-chemical vapor deposition technique. Under the same background pressure, the same H2 flow rate, by changing B2H6 flow rate and deposition time, we fabricate a series of ultra-thin films with thickness ranging from 5 nm to 80 nm. These films grow on SiC substrate, and are all c-axis epitaxial. We study the Volmer-Weber mode in the film formation. As the thickness increases, critical transition temperature Tc(0) also increases and the residual resistivity decreases. Especially, a very high Tc(0) 32.8 K for the 7.5 nm film, and Tc(0) 36.5 K, low residual resistivity (42 K) 17.7 cm, and extremely high critical current density Jc (0 T,4 K) 107 A/cm2, upper critical field Hc2(0) for 10 nm film are achieved. Moreover, by optimizing the H2 flow rate, we obtain relatively smooth surface of the 10 nm epitaxial film, with a root-mean-square roughness of 0.731 nm, which makes them well qualified for device applications.