Precise control over the size and morphology of the Au spiky nanoparticle (SNP) is essential to obtain narrow and tunable surface plasmon resonance (SPR). However, these challenges require a fundamental understanding of the particle growth mechanism and kinetics as well as its morphological transition, which can only be achieved by real-time observation at nanometer resolution. Here, we report in situ liquid cell transmission electron microscopy studies of single and multiple Au SNP growth at various conditions of such parameters as size and dose rate of electron beam and HAuCl4 solution concentration. The particle evolves via a mixture of reaction and Au formation-limited growth, followed by Au formation-limited growth-a transition from faceted to roughened surfaces, confirmed by the analysis with different beam sizes and the UV-vis spectra that feature a unique trend of short- and long-wavelength plasmon band shift. Quantitative analyses combined with a theoretical model determine the transition time (t(c)) of the two regimes and estimate the surface concentration (c(i)) of the particle with time. Interestingly, t(c) can be manipulated by the particle density, which alters the surface roughening rate, and the density is modulated by tuning the aforementioned parameters based on DLVO theory. These results suggest a new method for synthesizing a Au SNP whose size, morphology, SPR, and density can be sensibly manipulated without adding reducing or capping agents.