he optical, electronic and magnetic properties changes with the reduction of particle sizes, this is because the nano sized structures exhibits certain unique properties different from their bulk counterpart. Bandgap engineering for nano structures is one of the prominent area of research now. All though the bandgap increases with the reduction in the particle size, however there are several other ways by which the band gap can be changed. Fast crystallization method has been reported here and is compared with the conventional method. Absorption spectra are obtained using UV-vis optical spectrometer and from the absorption spectra tauc plot is drawn to find out the band gap theoretically. The band gap of nano particles and nano rods grown with the same nano particles as seeds are found to be different. The work shows a comparative study on the resultant bandgap for ZnO nano particles and ZnO nano rods grown with hydrothermal method. It is observed that the fast crystallization method results in decreases of band gap by around .8 eV compared to conventional method. Keywords:ZnO, nano particle, nano rod,bandgap