The structural phase transition induced by magnetic fields on the ferromagnetic compound MnAs was observed by the X-ray diffraction measurements in high magnetic fields up to 4 T. Magnetization measurements showed that Curie temperature T-C was 315.5 K for increasing temperatures and a metamagnetic transition from the paramagnetic to the ferromagnetic state occurred above T-C. The structural transformation from the hexagonal NiAs-type to the orthorhombic MnP-type structure was confirmed at T-C with increasing temperature from 290 K. The X-ray profiles at 319 K in the magnetic field showed that the hexagonal structure phase appeared with increasing fields and coexisted with the orthorhombic one. On further increase of magnetic fields, the single phase of the hexagonal structure was observed above 3.5 T in the forced ferromagnetic state. Both the magnetic and structural transitions induced by magnetic fields above T-C were first order with a hysteresis and had a close relationship between each other.