To clearly understand the spatial distribution and sea-to-air flux of non-methane hydrocarbons (NMHCs) and their environmental effects in the coastal seas of China, the concentrations of NMHCs (ethane, ethylene, propane, propylene, i-butane, n-butane, isoprene) were determined in the Yellow Sea and the East China Sea during March-April in 2017. Mean concentrations of oceanic NMHCs were 18.07 (ethane), 67.14 (ethylene), 15.35 (propane), 20.53 (propylene), 6.80 (i-butane), 6.41 (n-butane), and 17.00 pmol L⁻¹ (isoprene), respectively. The concentrations of oceanic NMHCs decreased with carbon numbers and the alkenes were generally present in higher concentrations than their saturated homologues. Except isoprene, no clear associations between Chl-a and other dissolved NMHCs were found in this cruise. Average atmospheric mixing ratios of individual NMHCs were 4.36 (ethane), 0.92 (ethylene), 3.45 (propane), 0.17 (propylene), 0.21 (i-butane), 0.94 (n-butane), and 0.53 ppbv (isoprene), respectively. Contrary to the seawater NMHCs, the concentrations of alkanes in the atmosphere were higher than those of unsaturated alkenes. Besides, atmospheric concentration of alkenes were higher in daytime than in nighttime, whereas alkanes displayed an opposite trend in the diurnal variation. Emission from ocean^’s surface into the atmosphere is considered to be the major loss of NMHCs. Based on the simultaneously measured atmospheric and seawater concentration, the sea-to-air fluxes of NMHCs were estimated to be 34.96 (ethane), 32.65 (propane), 262.54 (ethylene), 74.98 (propylene), 20.41 (i-butane), 17.41 (n-butane) and 54.95 nmol m^-2 d^-1 (isoprene), indicating that the coastal and shelf seas of China may be important sources of NMHCs to the atmosphere. For the first effort to estimate the contributions of the individual NMHCs to secondary organic aerosol (SOA) and ozone formation over the study area via SOAP (toluene weighted mass contributions) method and propylene-equivalent (Prop-Equiv) concentration, maximum incremental activity (MIR), respectively. The SOA concentrations formed from ethane, ethylene, propylene, n-butane, and isoprene were 3.2×10^-4, 8.1×10^-4, 2.8×10^-4, 4.0×10^-4 and 14.7×10^-4 µg m^-3, respectively. The ozone formation potential (OFP) of individual NMHCs was 1.64 (ethane), 10.37 (ethylene), 3.33 (propane), 3.72 (propylene), 0.67 (i-butane), 2.80 (n-butane) and 17.10 (isoprene) µg m^-3, respectively. The contributions of alkanes to SOA and OFP were lower than alkenes although alkanes concentrations were higher, with the highest contributor to SOA and ozone formation of isoprene. The results of this study have demonstrated that the control of NMHCs, especially alkenes such as isoprene, should be a focus of future regulatory measures in order to reduce air pollution over the study area.