The coupling between singlecore fiber and dualcore fiber is one of the major obstacles to the further study and application of dualcore fibers. With Gaussian approximation for fundamental mode in fiber, the expressions of energy coupling into either of the two cores, overall coupling efficiency between the two fibers, and the ratio of energy coupling between the two cores are deduced. The quantitative characteristics of mode matching, the distance between the two cores in dualcore fibers, influence of core position on coupling effect, are discussed in detail. Experimental result shows a good agreement with calculated result.