This paper deals with the experimental study on the variation in the load carrying capacity between concrete filled GFRP box beams of size 1200x150x200 mm is predicted by varying thickness of GFRP box beams as 4mm, 6mm and the concrete strength as M40. The material properties of cement, fine aggregate and coarse aggregate would be found out. The compressive strength of concrete cube would be found out to confirm the strength ? grade 40. Study results showed that in addition to many advantages due to its formation, the Box Beam showed superior physical and mechanical properties. It was found that the flexural strength and fracture toughness values of Composite beams significantly increased stiffness when compared to reference values. Flexural two point load would be applied on the box beams filled with plain concrete. The experimental test was performed to find the flexural strength, load carrying capacity, deflection, load deflection relationship, load strain relationship and stiffness ratio for various thickness of box beams. The analytical Study was performed by using ANSYS to evaluate the deformation of the specimen. The experimental study of beams showed that the box beam having higher thickness will increase the load carrying capacity and stiffness and also decrease the deflection. In ANSYS by varying both thickness of GFRP box as well as grade of concrete is analysed. The proposed finite element model shows increased resistance to deformation when concrete is used as infill material and the deformation decreases when the grade of concrete and thickness of box beam increases.