Undoped and transition metal (TM)-doped ZnO nanoparticles (Zn 0.98 X 0.02 O-NPs, X = Mn, Cr, Co and Fe) were synthesized from a metal nitrate precursor and gelatin by a sol-gel method. The compounds were synthesized at calcination temperatures of 550 • C for 6 h. The synthesized undoped/doped ZnO-NPs were characterized by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). XRD results indicated that the sample products were crystalline with a hexagonal wurtzite phase. SEM images showed the ZnO-NPs nearly spherical shapes and a non-uniform shape for doped ZnO-NPs. The crystalline development in the ZnO-NPs was investigated by X-ray peak broadening. The size-strain plot (SSP) method was used to study the individual contributions of crystallite sizes and lattice strain of the undoped and doped ZnO-NPs. The obtained results showed that strain of the NPs plays an important role in peak broadening; moreover, the mean crystalline size of the undoped and doped ZnO-NPs estimated from the SEM and the SSP method was highly inter-correlated. Finally, optical properties of the samples were studied by a UV-Vis spectrometer.