We study the effect of particle losses on dynamical phase diffusion in a two-mode Bose-Einstein condensate. Starting with an equal-population coherent state, the single-particle coherence is solved analytically by the Monte Carlo wave function method. We show that decoherence slows down the loss of single-particle coherence. Moreover, we find that the bigger phase diffusion is, the more sensitive to the noise it becomes. Finally, we discuss the effect of the losses on the spin squeezing and

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... that the squeezing angle is robust to the decoherence even though the value of the squeezing is greatly affected.