Dead band nonlinearity is one of nonlinearity errors in digital closed-loop fiber optic gyroscope (FOG). Output noise and drift of FOG can be reduced by attenuating dead band nonlinearity. Based on the definition of the resolution and threshold for FOG, a dead band definition and test method in digital closed-loop FOG were proposed. The frequency and phase characteristic of interfering signal and the process that the interfering signal tracked and locked input signal were analyzed to prove that electrical crosstalk between phase modulator and photodetector and modulation error were main interference sources causing dead band. A dead band model of digital closed-loop FOG was developed by combining an ideal digital closed-loop FOG model and a partial integration model of interfering channel, and a condition expression of dead band and an expression of output error produced by dead band were derived from the dead band model. The simulation results of the dead band are identical with the experimental results of the dead band, which indicate that the dead band model of digital closed-loop FOG is accurate.