Abstract: Background and Objective: To develop socio-economic in Northeast Thailand, growing of rubber trees are occur instead some cassava and sugarcane plantations. The environmental consideration respected for rubber plantation according to chemical fertilizer application for increasing of latex yield in the long-term. Soil fertility and variations of microorganism were investigated under rubber plantations in Northeast Thailand. Materials and Methods: A voronoi sampling method was used to collect soil samples from different ages (5, 11 and 22 years) of rubber in 3 areas. For each plot, 4 surface (0-10 cm) soil samples were randomly collected and composited. The soil samples were analyzed for chemical and physical properties and microorganisms. An analysis of variance was performed on data for each parameter with each treatment. Results: The study showed the soil pH, total N, exchangeable K and Ca, OM, NO3–, soil moisture and soil microorganisms tended to increase with the increasing age of the rubber trees, while the soil bulk density tended to decrease. Conclusion: Growing rubber trees as a monoculture in long-term resulted an improvement of some chemical and physical properties of soil and microorganisms. To conserve soil fertility under rubber plantations, the use of organic fertilizer combined with chemical fertilizers is recommended.