The influence of the pH-value of the aqueous solution of P123-micelles on the growth and formation of gold nanoparticles (AuNPs), which is of immense importance for their controlled growth in a simple single-step synthesis process, was investigated using time-evolution optical absorption spectroscopy, dynamic light scattering and transmission electron microscopy techniques. The size and structures of the AuNPs are found pH-dependent, even within basic region, with transition near pH ≈ 9.5, though the free P123-micelles remain almost unchanged. Below this pH-value, the slow reduction rate of gold ions creates less number of nucleation centers, which through autocatalytic thermodynamically controlled reduction (ATCR), initially formed chain-like aggregated small AuNPs (of different chain length) and subsequently through further diffusion and coalescence, formed well-faceted near symmetrical large AuNPs of size ≫ 19 nm, the size of the free P123miclles. Above this pH-value, the fast reduction rate of the gold ions creates large number of nucleation centers. The growth of which is well restricted by the limited amount of available gold ions for the ATCR and also due to the metal-polymer hydrophobic and polymer-water hydrophilic interactions. Accordingly, controlled growth of the majority of the centers takes place through ATCR, diffusion and early capping through near individual micelles to form isolated symmetric small AuNPs of size < 19 nm, with narrow size distribution, which are really intended for the different applications and fundamental studies. However, minority (but not insignificant amount) of the centers still remain in very small sizes and trapped inside the large micellar assemblies or even in the near atomic states, which create hindrance in the yield of the isolated small AuNPs.