Unintentional gallium incorporation was observed and investigated in the epitaxial growth of InGaN. The gallium incorporation rate was significantly influenced by temperature and TMIn source flow. The source of the unintentional gallium incorporation is confirmed to be from the flow distributor of the reactor. The gallium originates from transmetalation reaction between indium precursors and residual gallium. The unintentional gallium incorporation influences the growth of InGaN layers.