Abstract: In order to obtain the interaction mechanism between the grout and masonry in grouted concrete block masonry structures under vertical load, the hooping factor which denotes the hooping action of the masonry to the grouted concrete and the stress distribution in the grouted concrete block masonry is studied based on the minimum potential energy principle. The elastic modulus of grouted concrete block masonry can be approximately calculated according to the assembled section formula in material mechanics, and the calculation results of this formula agree well with the test results of 281 specimens in 65 groups. Another proposed formula to calculate the elastic modulus of the grouted concrete block is derived base on the test results by regression analysis method. Theoretical analysis and test results show that the value of elastic modulus of grouted concrete block masonry in our current code is low, indicating it is unsafe to guide the design of reinforced concrete block masonry structures.