In this paper, we found that very low concentration of iodide down to 8.8 nM had a good affinity to silver TNPs, and resulting in transformation of silver TNPs to nanoparticles and color changing of solution from blue to yellow. This process is different from the reported papers that etching and smoothing the corners of triangular nanoplates to circle nanoplates, but directly fusing thenanoplates to nanoparticles. After silver TNPs had been just fused by certain concentration of iodide completely, the color of the silver TNPs solution would show an intermediate color between blue and yellow. We defined the intermediate color as critical color and the concentration of iodide added as critical iodide concentration. We also found that there is a linear relationship between critical iodide concentration and silver TNPs concentration and thus,