  We study the decoherence of massive fields during inflation based on the
Zurek's density matrix approach. With the cubic interaction between inflaton
and massive fields, the reduced density matrix for the massive fields can be
calculated in the Schr\"odinger picture which is related to the variance of the
non-Gaussian exponent in the wave functional. The decoherence rate is computed
in the one-loop form from functional integration. For heavy fields with
$m\gtrsim \mathcal{O}(H)$, quantum fluctuations will easily stay in the quantum
state and decoherence is unlikely. While for light fields with mass smaller
than $\mathcal{O}(H)$, quantum fluctuations are easily decohered within
$5\sim10$ e-folds after Hubble crossing. Thus heavy fields can play a key role
in studying problems involving inflationary quantum information.
