Abstract: The gas diffusion (D_g) and permeability (K_g) coefficients are the two important parameters for the study on gas transport in unsaturated soils. In the present study, the theory of calculating D_g and K_g for unsaturated soils under steady state is derived. D_g and K_g of unsaturated soils are determined based on the relationship between tracer gas concentration (C) and gas flow rate (q₁) and that between pressure gradient (p) and gas flow rate (q₂), respectively. A device for in-situ measurement of D_g and K_g in unsaturated soils is designed and developed. The soil column tests are conducted to measure D_g and K_g of unsaturated soils with different water contents. Thereafter, the element tests and numerical simulation analysis are used to verify the effectiveness of the new method and device. It is found that D_g and K_g obtained by the new method are in good agreement with the results of the element tests and numerical simulations. The results of the sensitivity analysis show that the measurement accuracy of D_g reduces as the soil water content decreases, while the measurement accuracy of K_g is independent of water content of soil. The measurement accuracies of both D_g and K_g decrease with the increase of the radius of the aeration bulb, but the measurement of K_g is more sensitive to the change of the radius. The measurement accuracy of K_g is about 55% of the true value, when the radius of the aeration bulb is 0.05 m. The soil anisotropy reduces the measurement accuracy; when the values of D_gv/D_gh and K_gv/K_gh is 0.1, the measurement accuracies of D_g and K_g can still reach more than 40%.