The present work deals with the magnetic anisotropy of a textured Ni51.4Mn28.3Ga20.3 thin film sputter deposited on alumina ceramic substrate. This film is a ferromagnetic martensite with a strong uniaxial magnetocrystalline anisotropy. The magnetization versus magnetic field dependencies at different angles between the direction of magnetic field and the film plane are measured with a vibrating sample magnetometer. The initial magnetic susceptibility decreases with increasing inclination with respect to the film plane. A consistent procedure is introduced for the quantitative determination of (i) the angle of the easy-magnetic axis with respect to the film normal and (ii) the ratio between the magnetic anisotropy energy and the magnetostatic energy (quality factor). These values proved to be equal to 67°±4° and 2.9±0.3, respectively.