The effect of thin GaP insertion layers on the structural and optical properties of InP/In0.49Ga0.51P self-assembled quantum dots (SAQDs) on GaAs (001) substrate grown by metalorganic vapour phase epitaxy has been reported. The properties of InP/In0.49Ga0.51P SAQDs are modified when a thin (14 ML) GaP layer is inserted underneath the InP quantum dots (QDs). Deposition of the GaP insertion layer affects the dot dimension and improves the size uniformity. The density, dimension and uniformity of InP QDs strongly depend on the GaP insertion layer thickness. This variation in QD size is a result of a material nucleation effect caused by atomic intermixing between the InP QDs and underlying GaP insertion layer and surface energy. The insertion of GaP layer led to tuning the emission wavelength and narrowing of full width at half maximum (FWHM) when they are characterised by PL measurements at room temperature. (C) 2012 Canadian Society for Chemical Engineering