Gold nanoparticles possess a unique optical property, in which the color of colloidal solution changes as the properties of nanoparticles or surrounding medium change. Study on the so-called localized surface plasmon resonance (LSPR) of triangular gold nanoparticles is eminent, due to the growing interest in applying gold nanotriangles as colorimetric sensors. In this work, the size effect on the LSPR of triangular gold nanoparticles was studied by means of boundary element method (BEM) simulation. The dielectric constant of medium was set to reflect that of water, while the incident light was supposedly polarized in x and y direction. The simulation results were then used to evaluate the size evolution of triangular gold nanoparticles during laser-induced synthesis. The experiment was carried out by subjecting Au3+ ion solution to femtosecond laser irradiation for 5 s, before leaving it to grow for 24 h. The ion solution also contained 0.025 μM polyvinylpyrrolidone (PVP). Hydrogen peroxide (H2O2) was added shortly after laser irradiation to ensure complete reduction of Au3+ ion. During the post-irradiation period, the extinction was monitored hourly using UV-vis spectrophotometer. The simulation study using BEM revealed an exponential correlation between aspect ratio and extinction peak wavelength of gold nanotriangles. Using quasistatic approach, the extinction peak wavelength was expected to be independent from the nanotriangle height. Accordingly, only aspect ratio of a nanotriangle could be determined if the extinction peak wavelength was known. The dominant size of triangular gold nanoparticles formed during laser-induced synthesis showed an evolution as the autocatalytic growth progressed. The final aspect ratio (24 h after laser irradiation) was estimated to be 14.05. This result suggested the potential use of BEM simulation in studying the growth or decay of anisotropic nanoparticles.