Particle size and size distribution are two of the most vital parameters of nanomaterials since their physical properties as well as applications are highly dependent on these key factors. The objective of this work is to study the particle size and size distribution of gold nanoparticles (AuNPs), which were synthesized by a simple green synthesis in HAuCl4 and water-soluble chitosan solution (WSCS). Transmission electron microscope (TEM) and small-angle X-ray scattering (SAXS) techniques were carried out to determine the particle size, size distribution and polydispersity of AuNPs. The evidences from TEM and SAXS indicated that AuNPs can be formed in spherical shape with the size in the range of 10–30 nm. From TEM, the particle sizes of the AuNPs prepared in WSC, S at the concentration of 0.02 and 0.05 mg/mL containing 0.1 mM HAuCl4 were 20.5 ± 2.1 and 21.2 ± 1.8 nm, respectively. By increasing the concentration of HAuCl4 precursor up to 0.5 mM, AuNPs exhibited a slightly larger particle size (24.4 ± 2.2 nm). The particle size, size distribution and polydispersity of AuNPs observed by TEM and SAXS were in agreement. However, AuNPs sizes from SAXS were smaller than those from TEM because only the Au core of the AuNPs was detected by X-ray scattering. In addition, the chemical structure and specific surface plasmon resonance (SPR) absorption of AuNPs were well characterized by FTIR and UV-vis spectrophotometer. The FTIR results of AuNPs implied that the stabilization of AuNPs in the WSCS was due to electrostatic interaction and steric hindrance of C=O, C–O–C, –OH, –NH2 groups. By using WSCS at concentration 0.02 and 0.05 mg/mL, the SPR absorption bands were observed at 545 and 535 nm, respectively. The TEM and SAXS results of AuNPs in this study will provide an essential basis for synthesis of metallic nanoparticles for various applications in the future.