Based on longitudinal data from biennial surveys of small- and medium- sized enterprises (SMEs) in Vietnam conducted from 2007 to 2013, we find supports to the self-selection and learning-by-exporting hypotheses. We find that the SMEs having higher sunk costs and capital size are more likely to become exporters. Applying the propensity score matching method in combination with the Difference-in-Difference estimation, the study finds that export has raised SMEs’ productivity measured by either TFP or labor productivity, sales revenue, and value added of the SMEs. Furthermore, the gains from learning-by-exporting and specializing in core-competence products were stronger in the early years of entry into the export markets. These findings suggest policies to promote export of SMEs in an appropriate timing.