Abstract: Vesicular-Arbuscular Mycorrhizal (VAM) forms a symbiotic association with the root systems of 80% of angiosperms. These ubiquitous organisms can represent from 5 to 50% of the microbial biomass of the soil and give the host plants better mineral nutrition, a greater tolerance to abiotic factors as well as a protection accumulating against certain pathogens. These organisms represent a key functional group at the interface between the plant and the soil, contributing to the productivity and sustainability of forest ecosystems. The present work aims to examine the diversity of VAM in the soil of a natural cork oak forest and a cork oak forest invaded by Acacia mearnsii in North-eastern Algeria. The VAM spores were isolated by wet sieving method, then counted and characterized morphologically. In addition, the roots of the invasive species were sampled in the invaded site and examined to determine their colonization rates by (VAM). The results showed that the number of spores varied between the two studied soils. The soil invaded by A. mearnsii was more diverse in VAM morphotype and presented a higher number of spores (146 spores/100 g of soil) compared to the non-invaded soil (53 spores/100 g of spores). Regarding the rate of mycorrhization, the roots of A. mearnsii were more than 80% colonized.