We have studied the two-state random decoherence model of the V15 system (Phys. Rev. Lett. 84 (2000) 3458) with the 'partial tracing of the environment' method. The linearity relation between the averaged decoherence time and the random dipolar interaction amplitude is verified. The distribution function of decoherence time spreads more widely as the amplitude of the random field is decreased, hence it is difficult to define a decoherence time for the system.