Monometallic nanoparticles of silver, Ru, Ni and bimetallic nanoparticles of Ru-Ni were synthesized by spray pyrolysis. Concentration of ruthenium chloride and nickel chloride was varied in the precursor solution to synthesize the nanoparticles with core and shell morphology. However, all the conditions did not generate core-shell morphology. The operating range in which the ratio of ruthenium chloride, nickel chloride was varied in combination with temperature was critical in determining the particle morphology and structure. Particle size reduction with precursor concentration in monometallic silver nanoparticles and bimetallic Ru-Ni nanoparticles was studied using two theoretical approaches. The results of the nucleation approach and Navier-Stoke's approach were found to be similar. Theoretical results matched well with the experimental results. Particle size was found to decrease with precursor concentration until a critical particle size was reached. Any further increase in concentration beyond this point did not have any impact over the particle size. Using both the approaches in case of monometallic and bimetallic systems proved this. Particle size reduction with precursor concentration was found to follow an exponential curve.