This paper presents an experimental program to investigate the flexural behavior of hollow reinforced concrete encased steel tube composite beams. Experimental results from nine hollow reinforced concrete encased steel tube composite beams are provided in this paper. The beam dimensions were 250x600 mm & 250x480 mm cross section (6100 mm long) with variable hollow rectangular steel tube with variable spans. The beam ends were filled with concrete to form solid end diaphragms to prevent local distortion. The beams were subjected to bending. Hollow reinforced concrete encased steel tube composite beams possess excellent flexural resistant properties such as high strength and ductility. The objective of this paper is aimed at investigating the behavior of Hollow reinforced concrete encased steel tube composite sections. Various available codes and literatures were compared for this purpose. Based on the analytical program an experimental programs are proposed to be performed. In the present investigation, an experimental study has been carried out. The main objective of this study is to find out the ultimate moment carrying capacity of hollow reinforced concrete encased steel tube composite members (CEST). Concrete encased steel tubes (CETs) are efficient members in structural applications including buildings & bridges, and their use in the building industry is increasing. To date their primary use has been in axial applications, with the design methodology based on theory and tests of columns under loads applied axially or at relatively small eccentricities. Limited in to research into the behavior of CETs subjected to large eccentricities or loading in pure flexure have been conducted, with preliminary experimental investigation suggesting that stiffness of composite members tending towards that of the tube at relative low loads. An ongoing research program on the flexural behavior of CETs is being undertaken on thin-Hollowed steel tubes that can provide an economical form of construction. In this paper, the results from experimental work will be presented with a particular emphasis on the stiffness of the members when subjected to flexural loading. The experimental work to date has demonstrated that even in composite specimens where there is negligible end slip, the stiffness of the specimens under pure flexure tends to the stiffness of the bare steel tube. From the experimental results some guidelines for the design of concrete encased tubes subjected to primarily flexural loading will be presented.