This paper investigates the role of transport in shaping the impacts of urban agglomeration on firm productivity across countries, through a meta-analysis of 1331 reported estimates of productivity elasticity of urbanization from 50 primary studies. Using the framework of Melo et al. (2009), we specially focus on the impacts of national transport networks and transport use on the agglomeration elasticity estimates. The meta-regression results indicate that the elasticity estimates of a country increase with its transport network and transport use density, even after the effects from different patterns of city size and spatial distribution are corrected for. Also, study characteristics do matter, especially the specification of spatial interaction effects and sources of agglomeration economies, and choice of estimation methods. This meta-analysis partially opens the black box and provides a better understanding towards the cross-country heterogeneity of urban agglomeration economies.