We report the synthesis and optical properties of compact and aligned ZnO nanorod arrays (dia, ~ 50-200 nm) grown on a glass substrate with varying seed particle density. The suspension of ZnO nanoparticles (size, ~ 15 nm) of various concentrations are used as seed layer for the growth of nanorod arrays via selfassembly of ZnO from solution. We studied the effect of various growth parameters (such as seeding density, microstructure of the seed layer) as well as the growth time on the growth and alignment of the nanorods. We find that the growth, areal density and alignment of the nanorods depend on the density of seed particles which can be controlled. It is observed that there is a critical density of the seed particles at which nanorod arrays show maximum preferred orientation along [002] direction. The minimum and maximum radius of the aligned nanorods synthesized by this method lie in the range 50-220 nm which depend on the seeding density and time of growth. These nanorods have a bandgap of 3⋅3 eV as in the case of bulk crystals and show emission in the UV region of the spectrum (~ 400 nm) due to excitonic recombination and defect related emission in the visible region.