Classically, the grain size of sediment samples is determined by the sieve method for the coarse fractions and by the pipette method, based on the Stokes’ sedimentation rates, for the fine fractions. The results from the pipette method are compared with results from laser diffraction size analysis using a set of randomly selected 106 surface sediments which are collected from the Eastern South China Sea. The aim of this research is to calibrate systematically the grain size and content of clay fraction and sediment type achieved from laser diffractometry. The contents of sand, silt, and clay in sediments of eastern South China Sea with depth more than 2 000 m were 3.7%, 42.4% and 53.9%, respectively, obtained from standard sedimentation method, and 9.3%, 61.1%, 29.6%, respectively from laser diffractometry.