More and more counties in Iowa have experienced a serious population loss from out-migration. The resulting population decline has worsened existing problems such as insufficient education, limited infrastructure, and difficulties of access to health care. This study will use a modified version of the community capitals framework to better understand what community factors affect in- and out-migration in Iowa small towns. The study uses data from a survey of 99 small Iowa communities combined with information from the American Community Survey in 2014 to examine the extent to which community attachment and four community capitals (social capital, financial capital, human capital, and built capital) affect migration. The results based on structural equation models show that the migration models (in-, out-, and net-migration) do not fit the data. Three aspects of social capital (bonding social capital, bridging social capital, and trust) are the effective predictors of community attachment. Financial capital is the most important factor affecting in-migration and out-migration. Community attachment is the second most important factor reducing out-migration. Built capital contributes to the increase of social capital, which in turn facilitates the strength of community attachment and therefore reduces out-migration. Although communities with higher human capital have higher out-migration, these communities attract other new residents to move in. The effect of community capitals and community attachment on migration varies by ages. Community attachment has more effects on young people (1 to 17 years) and old people (65+); financial capital has more effects on young adult (18 to 34 years) and old adult (35 to 64 years).