We have successfully grown single-crystalline ferromagnetic MnAs thin films on (001) GaAs substrates by molecular beam epitaxy. By reflection high energy electron diffraction and x-ray measurements, the growth direction of the MnAs thin films was found to be [1100BAR] on (001) GaAs, and the epitaxial relationship was [0001] MnAs //[110BAR] GaAs and [1120BAR] MnAs //[110] GaAs. Magnetization measurements at room temperature have revealed that the epitaxial MnAs thin films have strong magnetic anisotropy, and that the easy magnetization direction is in-plane, along the [1120BAR] axis of the MnAs thin films which is parallel to the [110] axis of the GaAs substrate, with almost perfect square hysteresis loops, relatively high remanent magnetization, and low coercive field.