Traditionally the euphotic zone is defined as a layer from surface to a depth (ZeuPAR) the photosynthetically available radiation (PAR) is 1% of its value at the surface; while the euphotic zone in biological term is defined as a layer from surface to a depth (ZeuNPP) there is no net production. As a result, ZeuPAR can be very different from ZeuNPP, and there has been debate for decades which one should be used to represent “euphotic zone”. In this study, based on measured profiles of radiation and primary production in the South China Sea, the compensation irradiance (Ic) corresponding to ZeuNPP is compared with PAR value at depth, as well as the value of  usable solar radiation (Lee et al. 2014). It is found that the ratio IcPAR/PAR(0) is generally ~0.52% (± 0.13%), while the ratio IcUSR/USR(0) is generally ~0.98% (± 0.08%), which indicates ZeuPAR could be too shallow to represent the “euphotic zone”, at least for these subtropical waters. On the other hand, the depth of 1% surface USR (ZeuUSR) appears closely matching ZeuNPP. Furthermore, the positions of all subsurface chlorophyll maximum are found slightly above ZeuUSR, but can be much deeper than ZeuPAR. These results suggest that ZeuUSR could be a good candidate to unify the concept and determination of the “euphotic zone”, at least for such subtropical waters.