Gold nanoparticles are versatile material, which is among many others applications also used in experimental cancer treatment where gold nanoparticle`s phototermal effect [1] is employed. For that reason gold nanoparticle`s nucleation processes and chemical reaction kinetics need to be well understood in-situ in liquid at the elevated temperature conditions [2]. In our experiment 1.5 mM water solution of gold(III) chloride trihydrate (chloroauric acid) as source of gold was used. The experiment was performed using Jeol JEM 2100 transmission electron microscope (TEM) operated at 200 kV combined with the liquid cell. The sample was observed at an electron dose of 108 Gy/s. Firstly the sample was observed for several minutes at the room temperature (25 °C), proving the solution is stable at these conditions. After that the sample was heated to 50 °C. After 30 s 5-10 nm sized nanoparticles (NPs) started to appear. The NPs grew for a period of 2.5 min and at this point reached the final size of 30-60 nm. After that gold monomers grow around NPs for further 3 min. In the final the stage: NPs particles did coalesce in a larger individual clusters. Selective electron diffracting analysis proved the face centered cubic crystal structure. We are suggesting that the formation of gold nanoparticles is a 4-step process. First step is the formation of seed particles, which could be explained by partial reduction of the gold precursors. This is followed by the growth of the golden hydroxide around the seed particle. In the third and fourth step all the reaming gold is reduced, which results in the inside-out growth and coalescence of the NPs.