We have studied physical properties of MnAs thin films grown by Molecular-Beam Epitaxy as well as their post-growth annealing effects. The samples grown at $600^{circ}C$ show the preferred crystal orientation of type-B independent of substrate whereas type-A is observed for the samples grown at below $200^{circ}C$. The sample grown at $600^{circ}C$ on GaAs(001) substrate is magnetized to only one direction even on the easy axis of magnetization. The magnetic properties are vastly enhanced after post-growth annealing for both MnAs/Si(001) sample with no ferromagnetism and ferromagnetic MnAs/GaAs(001) grown at $200^{circ}C$.