In magnetic recording, the recording fields are essentially vectorial, and the magnetization process in the recording medium has to be analyzed using vector magnetization. From this fact, a vector magnetization distribution in the recording medium must be evaluated by both magnitude and direction of magnetization. This paper describes the vector magnetization distributions obtained by a new method, using reversible and irreversible rotational magnetization model of single domain acicular particles with uniaxial anisotropy. Calculations are done self-consistently at an instant when the head field is applied and after it is removed. Although the results are, at present, limited to the case where the recording medium is standing still, they show quite good agreement with the results of scaled up model experiments, and can clearly explain the demagnetization mechanism in terms of the vector rotation. This new calculation method will, in principle, display its real power in analyzing the dynamical recording process when the recording medium is moving along the head or the head field is changing.