Formation of gold nanoparticles (AuNps) in a single step, from aqueous solution of Au-salt of different concentration and fixed amount of P123 triblock copolymer as reducing and stabilizing agent, is studied using complementary techniques. Growth of AuNps from AuCl4- ions through ligand-to-metal charge transfer complex with time is evident from optical absorption study. The size of the AuNps increases with the Au-salt concentration, while the amount of AuNps first increases then decreases, suggesting important role of Au-salt and polymer ratio in the AuNps formation. Chain-like structure consisting of AuNps seems to form. The length and relative number of the chains decreases with time and also Au-salt concentration. Growth of large size aggregates from small size P123 micelles due to the loading of AuNps is evident from dynamic light scattering study. Formation of AuNps in the solution is confirmed using X-ray diffraction technique overcoming the problem of low scattering intensity.