The political and social system involved in the limbo between tradition and modernity of the Qajar Era caused widespread and quick developments in Iranian culture, art, and architecture as well as a challenge between tradition and modernity. Relatively numerous palaces built in this era were signs of greatness and distinction-seeking of the Qajar kings and aristocrats, who embraced all transformations of architecture in their era and played a significant role in changing Iranian architectural values. Therefore the purpose of the present study is to investigate developments in architecture of the Qajar Era and their effects on the evolution of palaces in this era. Using case studies methodology and referring to historical sources, contemporary studies on Qajar architecture and field studies conducted on outstanding palaces of this era, it can be concluded that the continuation of Safavid architecture early in this period, Qajar kings' tendency to Iranian ancient architecture, and the culmination of the architectural influence of the West in the Nasserid Era were the main factors effective on the formation of Qajar palaces. Moreover, the evolution of these buildings has occurred in three pre, early and late Nasserid Eras with various traditional, eclectic and European characteristics regarding the construction period and the effects of the above factors on the evolution of plans and circulation systems, facades and sections, materials, decorations, and architectural elements. In addition to shedding more light on common features of architecture and decoration of Qajar palaces, the study tried to determine their general and specific differences.