Arbuscular mycorrhizal fungi (AMF) can help host plants acquire nutrients and water from soil and therefore can enhance plant growth. Although AMF can change root traits of host plants, whether the effects of AMF on growth correlate with the effects on root traits is not well known. Here we performed a meta-analysis to quantify the effects of AMF on growth and root traits of dicotyledonous plants, and the correlation between these two effects under different soil phosphorus (Olsen-P) levels. AMF significantly increased total biomass, shoot and root biomass, length and area of roots, but decreased the specific root length. These AMF effects were influenced by soil P levels. Effects of AMF on plant growth were higher under low and intermediate soil P than under high soil P level. For root traits, however, the AMF effects were higher under intermediate soil P than under low and high soil P level. Regression correlation analysis showed a high association between the effect of AMF on plant growth and the effect on length and volume of roots. The correlation between effect of AMF on root length and mycorrhizal growth response was higher under low soil P level than under low and intermediate soil P level. The results suggest that the AMF effects on plant growth could be through altering the root traits, further affecting carbon allocation and water and nutrient acquisition.