This study is on the early historical background and the process of development of modern architecture in Thai, during the period from 1782(King Rama 1) to 1934(King Rama 7). Thai started tn form a connection with western nations from the late 18C, founded the Bangkok Dynasty. Since then, Thai was increasingly influenced by western civilization and this trend included an increasing influence of western architecture. In this paper, the centeral objective is to observe the architectural change during the period from King Rama 1(1782-1809) to King Rama 7(1925-1934). This can be divided into three period. During the first period, from King Rama 1 to King Rama 3(1824-1851), Thai architecture showed a tendency to follow the preceding traditional example and to imitate Chinese architecture. The second period is from King Rama 4(1851-1868) to King Rama 5(1868-1910). During this period, Thai architectural design was increasingly influenced by western concepts, specially European Neo-classicism Style. During the third period, from King Rama 6(1910-1925) to King Rama 7(1925-1934), Thu had been enjoying extensive commerce with western nations. The great developments in the field of architecture during this reign were apparent in the construction of public utilities and facilities. These buildings were designed by western architects. At this time, modern architectural concept of western was introduced.