  The boundary contribution of an amplitude in the BCFW recursion relation can
be considered as a form factor involving boundary operator and unshifted
particles. At the tree-level, we show that by suitable construction of
Lagrangian, one can relate the leading order term of boundary operators to some
composite operators of N=4 super-Yang-Mills theory, then the computation of
form factors is translated to the computation of amplitudes. We compute the
form factors of these composite operators through the computation of
corresponding double trace amplitudes.
