The increase of bit density is a critical point in the magnetic storage media. We intend to manipulate the easy axis in ferromagnetic thin films with ion irradiation technique. Co/Pt multilayered films were prepared for the study and then were ion-irradiated while an external magnetic field was applied to them. In consequence, we get a CoPt alloy film with an in-plane easy axis and change its direction with an external magnetic field applied during the irradiation. The energetic ion beam let the ferromagnetic film have a new atomic and magnetic structure. 1 Introduction Magnetic recording system is one of the most rapidly developing field of high technology in the world today. One of the most critical point for it is raising the recording density at which information can be stored in a media. This is concerned with the signal to noise ratio and the stability of smaller bits. In recent years, the areal density of the magnetic storage media, especially of the platter in hard disk drives, has been increased at a rate of up to 60% per annum [1 -4]. As the bit density of them gets much higher, that is, the bit size does even smaller, magnetic easy axis manipulation has attracted a lot of attention in the technology [5,6]. In common uniaxial recording, magnetic dipole interactions between neighboring bit areas may lead to a reversal of data bits and cross track interference that limit the ability to accomplish ultrahigh storage density [1,7]. Since the interaction energy between adjacent domains is proportional to the cosine of the angle between magnetization directions, it can be reduced by making the direction of adjacent anisotropies relatively orthogonal.In this paper, we attempt to make an in-plane easy axis in a ferromagnetic thin film that will be indispensable for the media with ultrahigh recording density, and then change its direction to the designed one. We choose a Co/Pt multilayered film for the sample, since it has good magnetic properties and its alloy is a main constituent of the platter in hard disk drives [8 -11]. Ion irradiation, our method, has the advantage in forming a metastable alloy, which has different characteristics from that of thermodynamically stable one, by putting inert ions in the multilayered film. We can call it the ion-beam-mixing process. When energetic ions with energy of several tens of keV penetrate a thin film, the collision cascades cause displacements of the target atoms, produce a new mixed phase through the intermixing of cobalt and platinum sublayers [12,13], and then get a metastable physe as a result of rapid quenching [14,15].