Correlation between growth mode transition and photoluminescence (PL) properties of Si1−xGex/Si quantum wells (QWs) with high Ge composition grown by gas‐source molecular beam epitaxy is investigated. With increasing Ge composition in the QWs, significant deviation from the theoretical calculation is observed for x≳0.4 in the emission energy and the activation energy obtained from the temperature dependence of integrated PL intensity. Transmission electron microscopy reveals emergence of islands for x≳0.4, implying strong correlation with the anomalous PL properties. Observed PL properties are shown to be well explained by the breakdown of two‐dimensional growth. An attempt to grow luminescent QWs with flat interfaces is also presented.