In this work we describe the synthesis, micro structure (XRD, SEM, AFM) of magnesium oxide nanoparticles and magnesium oxide thin films synthesized by urea-based combustion method and solution growth route using magnesium nitrate as the source of Mg. We used fuel-to-oxidizer ratio (psi) as a control parameter to investigate how lattice parameter, particle size, and micro strain vary with psi = 0.25-2 in the steps of 0.25. Earlier we have studied NiO as a substitutional solute in MgO (Rao KV, Sunandana (2005) Solid State Phys 50:235). The average crystalline size of MgO was estimated from the full width half maximum (Gaussian and lorentzian fits) of the X-ray diffraction peaks using Sherrer's formula and Williamson-Hall plot. The particle size varies from 15(+/- 0.3) nm to 60(+/- 1.2) nm as psi is varied systematically. Surface areas of the MgO powders measured using BET method were used to calculate the particle size, which is comparable with the crystalline size calculated from XRD. We also calculated porosity and microstrain in the MgO nanoparticles with varying psi. Thin films of MgO are well characterized from XRD and AFM. The size of the particles and RMS roughness of the thin films were calculated using AFM.