Random lasers result from the amplification of localized modes in random gain medium, and the amplification property depends on the spatial profile of the localized modes. Through the introduction of optical gain into system with an imaginary part of the complex refractive index, the amplification properties of localized modes are investigated by the finite-difference time-domain (FDTD) method. We find that different localized modes have different mode areas which have direct influence on the amplification property of the localized modes. The modes with smaller mode area undergo lower loss of energy than those with larger mode areas, so that they are amplified in advance while those modes with larger mode areas are suppressed due to mode competitions. This will lead to preferred amplification of the desired localized modes.