The epoxidation of propylene to propylene oxide is one of the most challenging research targets in catalysts. This review highlights our recent studies on the epoxidation of propylene by nitrous oxide and oxygen catalyzed by iron- and copper-based heterogeneous catalysts. For both iron- and copper-based catalysts, the modification with an alkali metal ion (especially K+) plays pivotal roles in obtaining high selectivity to propylene oxide. Alkali metal ions may enhance the dispersion of iron and copper species, change their coordination environments, and regulate the surface acid and base properties, and thus contribute to the selective formation of propylene oxide. The unique combination of active metal component (iron or copper) and oxidant (nitrous oxide or oxygen) is also crucial for the epoxidation of propylene. We propose that the oxidant is activated on active iron or copper sites with peculiar structures and oxidation states, forming active oxygen species responsible for the epoxidation of propylene.