Atmospheric CH_4 and N_2O are two most potent long-lived greenhouse gases that contribution to global warming. Sources and sinks of CH_4 and N_2O and the processes driving their spatio-temporal heterogeneity are critically important to global change. Understanding the diel variation in CH_4 and N_2O emissions is of importance to estimating CH_4 and N_2O budgets in large spatial and time scales. In this study, diel variations of CH_4 and N_2O fluxes in coastal salt marsh that invaded by Spartina alterniflora and vegetated with or without native Phragmites australis and Suaeda salsa were measured in site by using static chamber technique. Our results showed that aboveground biomass in S. alterniflora marsh was 1.70 kg·m~(-2) and SOC concentration was 13.55 g·kg~(-1), which were 2.50~3.43 and 2.15~4.15 times, respectively, those of P. australis and S. salsa marshes. Soil radox potential at 10-cm depth showed apparent diel variations in S. alterniflora and P. australis marshes, which were both highest at 12:00 am and lowest at 3:00 am. However, no significant diel variations of radox potential were found in S. salsa marsh, bare tidal flat and open water. Mean CH_4 flux was 0.52 mg·m~(-2)·h~(-1) in S. alterniflora marsh, which was 2.14~6.40 times those in other sites. Mean N_2O flux was-3.24 mug·m~(-2)·h~(-1) in S. alterniflora marsh and was significantly lower than those in S. salsa marsh, tidal flat and open water. CH_4 fluxes peaked at 0.73 and 0.30 mg·m~(-2)·h~(-1) at 15:00 and showed the lowest of 0.37 and 0.17 mg·m~(-2)·h~(-1) at 3:00 in S. alterniflora and P. australis marshes, respectively. CH_4 fluxes were negatively (P<0.05) correlated with porewater CH_4 concentrations in S. alterniflora and P. australis marshes and it was positively (P<0.05) correlated with soil temperature and redox potential at 10 cm and net ecosystem CO_2 exchange (NEE) in S. alterniflora marsh. N_2O fluxes were negative at 9:00-18:00 and positive at 21:00~6:00 and were negatively correlated with NEE in S. alterniflora and P. australis marshes. No discernible diel variations were observed in the CH_4 and N_2O fluxes in S. salsa marsh, tidal flat and open water. Our results suggest that S. alternniflora invasion stimulates CH_4 emissions but decreases N_2O emissions from Chinese coastal salt marsh, and the extent of diel variations in CH_4 flux depend on the gas transport capacity of plants, and capacity of transport O_2 and organic substrate supply down to the rhizosphere by plants may drive the diel variations in N_2O exchanges.