Arbuscular mycorrhizal fungi form obligate symbiotic associations with most of plant families. This limits their in vitro culturing and large-scale production. In vitro root organ culture is very useful for studying these symbiotic relationships and for mass production of arbuscular mycorrhizal fungal inoculants. This research aimed to study the effect of different phenolic compounds on the growth of the arbuscular mycorrhizal fungus Gigaspora gigantea using in vitro tomato root organ culture. Eight phenolic compounds were used against control without phenolic compounds. The phenolic compounds used in this research were cinnamic acid, catechin anhydrous, protocatechuic acid, ferulic acid, tannic acid, coumarin, esculetin and catechol. The experiments were done at two different pHs (5.7 and 6.5) in both solid and liquid media. Phenolic compounds exhibited different effects including stimulatory, inhibitory or no effects. Catechin anhydrous (which is a flavonoid compound) showed the most significant increase in both mycorrhizal root colonization and arbuscular abundance with moderate growth of root hairs at pH 6.5. Tannic acid inhibited the growth of root hairs, mycorrhizal colonization and formation of arbuscules at pH 5.7. Solid media were superior to liquid media in both mycorrhizal colonization and arbuscular formation.