In this study, life cycle of arbuscular mycorrhizal fungus, Glomus intraradices isolated from wheat rhizospheres of Damghan region in the Iran was observed using carrot (Daucus carota L.) transformed hairy roots in vitro. Mycorrhizal spores and roots were used as inoculum. Spore germination was observed 3-5 days after surface sterilization as well as co inoculation with transformed roots. Fungal growth was also recorded 2-10 days after inoculation. Fungal germinating hyphae branched and produced radical shape network 2.5 mm in diameter. The first contact between fungus mycelium and roots occurred 1-3 days after germination. 7 days after fungus-host contact, several secondary spores or vesicle like structures observed were similar to true spores except of their size (20-30 µm diameter). The first true spore formed 25 days after contact and then number of spores increased exponentially. These spores were hyaline and whitish at first but then turned to brownish yellow. After 12 weeks, 1000-2500 spores could be recorded in each plate.