We examined the time dependence of the relative magnetization (Mrela) and the relative permanent magnetization loss (Mloss) due to the oxidation in data storage tape prepared from ultrafine metal particulate (MP) composite in the preserving temperature range from 295 to 348 K for 1000 h. We also investigated the optimum stabilized condition at 295 K for 5000 h after demagnetization by initially applying the reverse field. The time dependence of the change in Mrela with no reverse field and the Mloss of MP tape were negligible at 295 K for 1000 h. However, the value of Mloss at above 338 K increased rapidly after 100 h had elapsed. It may be closely related to the oxidation of MP particles. The change in (Mrela-Mloss) was proportional to the logarithm of time for 1000 h at 295 to 348 K. The magnetization value at 295 K could be stabilized over 5000 h by initially applying the reverse field. During the time dependence of magnetization obeyed a logarithmic law, the optimum demagnetizing field for stabilization agreed very well with the critical field of magnetic aftereffect.