In this article, the overall phase transition behavior of a Ge-Te-Ag system prepared by co-sputtering Ag and pure GeTe was investigated over a wide Ag composition (0-39.3 mol%). Crystallization temperature was determined through an electrical resistance analysis. The crystalline phase was analyzed with X-ray diffraction measurement, selected area electron diffraction patterns, and high resolution transmission electron microscopy to study the effects of Ag incorporation into GeTe. Optical static tests were conducted to measure the energy of crystallization, and a crystallization kinetics study was also performed using the Johnson-Mehl-Avrami model. Optical band gap was also measured using a UV-Vis-NIR spectrophotometer. We found that the system displayed a compositional threshold behavior, wherein crystallization temperature first decreased with increasing Ag content and then increased with further increases in Ag content. Crystallization kinetics and crystallinity also showed the compositional threshold behaviors at the critical Ag composition. From understanding of the origin of these results, we expect this research to provide a means of manipulating intrinsic characteristics of phase change materials to achieve targeted performance.