Hydrocarbon gas often contains some amounts of heavier hydrocarbon and non-hydrocarbon components that contribute to its properties (i.e. viscosity and density). Prediction of the density and viscosity values for hydrocarbon gases is necessary in several hydrocarbon gas engineering calculations such as the calculation of gas reserves, gas metering, gas compression, estimating the pressure gradient in gas wells and for the design of pipeline and surface facilities. Literature correlations for the density and viscosity of pure hydrocarbon gas such as methane, ethane, propane, butane and isobutene are available. However, wide-ranging and accurate correlations for predicting the gas viscosity and density are not available for gas Mixtures associated with heavier hydrocarbon components and impurities components Such as carbon dioxide, nitrogen, helium and hydrogen sulphide. This paper presents two new models for estimating the density and viscosity of pure hydrocarbon gases and hydrocarbon gas mixtures containing high amounts of pentane, plus small concentrations of non-hydrocarbon components (i.e. carbon dioxide, nitrogen and helium), over a wide range of temperatures and pressures oil the basis of fuzzy logic approach. The density model was developed using apparent molecular weight, pseudo-reduced temperature and pseudo-reduced pressure. However, the viscosity model was developed using density, apparent molecular weight and pseudo-reduced temperature. The Fuzzy models were derived from 5,350 measurements of density and viscosity of various pure gases and gas mixtures. The partitioning of the input space into the fuzzy regions, represented by the individual rules, was obtained through fuzzy clustering. Accuracy of the new fuzzy models was compared to various literature correlations by blind tests using 1,460 measurements of density and viscosity. The results show that the new fuzzy models are more accurate than the compared correlations.