[en] Highlights: • Lattice strain of 4 nm sized ZnO nanocrystals by WH method is estimated. • (100), (002), (101) and (200), (112), (201) XRD line profiles are merged. • The merging of XRD lines makes WH analysis difficult. • The difficulty of merged XRD lines is overcome by Rietveld refinement technique. • Optical properties of the 4 nm sized ZnO are also presented. We apply Williamson-Hall (WH) method of X-ray diffraction (XRD) line profile analysis for lattice strain estimation of small sized ZnO nanocrystals (crystallite size≈4 nm). The ZnO nanocrystals are synthesized by room temperature chemical co-precipitation followed by heating at 40 °C. Zinc acetate, sodium hydroxide and 2-mercaptoethanol (ME) are used for the synthesis of the nanocrystals. {100}, {002}, {101} and {200}, {112}, {201} line profiles in the XRD pattern are significantly merged, therefore determination of the full width at half maximum values and peak positions of the line profiles required for WH analysis has been carried out by executing Rietveld refinement of the XRD pattern. Lattice strain of the 4 nm sized ZnO nanocrystals is found to be 5.8×10⁻³ which is significantly higher as compared to the literature reported values for larger ones (crystallite size≈17–47 nm). Role of ME as capping agent is confirmed by Fourier transform infrared spectroscopy. The band gap of the nanocrystals is determined from the UV–Visible absorption spectrum and is found to be 3.68 eV. The photoluminescence spectrum exhibits emissions in the visible (408 nm-violet, 467 nm-blue and 538 nm-green) regions showing presence of zinc interstitial and oxygen vacancy in the ZnO nanocrystals.