An experiment was designed to measure nail strength of metal-plate-connected joints under forces parallel and vertical to wood grain.SPFⅡ dimensional lumber with living or dead knots and domestic truss plates were selected.The results showed that the living knots enhanced the nail bearing capacity,while the dead knots impaired the nail bearing capacity when the percentage of invalid tooth number exceeded 15%.Under the force parallel to wood grain,the nail bearing capacity of the joints with dead knots was even decreased when the percentage of invalid tooth number was more than 11%.