Adjusting and controlling the relative humidity (RH) inside package is crucial to ensuring the quality of modified atmosphere packaging (MAP) of fresh produce. In this paper, an improved kinetic model for predicting the RH in MAP was developed. The model was based on heat exchange and gases mass transport phenomena across the package, gases heat convection inside the package, and mass and heat balances accounting for the respiration and transpiration behavior of fresh produce. And then the model was applied to predict the RH in MAP of Fresh lentinula edodes (one kind of Chinese mushroom). A set of testing system was established to obtain the gas composition and relative humidity-temperature in MAP. Fresh lentinula edodes were packaged in different initial gas composition and sample mass, and stored for 40h at 8℃, 80% RH. The parameters of the respiration model for lentinula edodes were estimated, and the RH and temperature inside packages were measured during storage. The model equations were solved numerically using Adams-Moulton method to predict the RH in model packages. In general, the model predictions agreed well with the experimental data, except the model predictions were slightly high in the initial period. The effect of the initial gas composition on the RH in packages was notable. In MAP of lower oxygen and higher carbon dioxide concentration, the ascending rate of the RH was reduced, and the RH inside packages were saturated slowly during storage. The influence of the initial gas composition on the temperature inside package was not much notable.