We studied seasonal distribution of the microphytobenthos and their primary production with $C^{14}$ method and carried out pigment analysis with HPLC in an estuarine mudflat of the Ganghwa Island, Korea from May 2002 to April 2004. The abundances of microphytobenthos were higher at the middle than upper part and lower part of intertidal flat. Abundances of microphytobenthos ranged from $2.3{ imes}10^5;cells;cm^{-2}$ to $140.9{ imes}10^5;cells cm^{-2}$. The bloom of microphytobenthos was observed in the early spring and then it decreased from spring to summer and autumn. The pennate diatom was a predominated group among the microphytobenthos in this area. The dominant species were Paralia sulcata, Cylindrotheca closterium and Nitzschia sp.. Nitzschia sp. and Cylindrotheca closterium were predominant in February. The results of pigment analysis suggest the presence of diatoms, euglenophytes, chlorophytes, cyanobacteria, cryptophytes, chrysophytes, prymnesiophytes, dinoflagellates and prasinophytes. The biomass of microphytobenthos ranged from 1.18 to 34.25 mg chl-a $m^{-2}$, with a mean of 7.60 mg chl-a $m^{-2}$. The mean ratio of Fuco/Chl a was 0.7 which indicates that most of biomasses of microphytobenthos were due to diatoms. The ratios of Chl b/Chl a ranged from 0 to 0.82(with a mean of 0.17), implying that euglenophytes and chlorophytes lived together in special period seasonally. Temporal variation of primary production ranged from 4.2 to 113.0 $mgC{cdot}m^{-2}{cdot}hr^{-1}$(mean value was 33.9 $mgC{cdot}m^{-2}{cdot}hr^{-1}$ and initial slope$({alpha})$ was measured from 0.002-0.005$(mgC;mgchl-a^{-1};hr^{-1}){cdot}({mu}E;m^{-2};s^{-1})^{-1}$. Assimilation number$(P_m)$ was in the range of 0.50-1.32 $mgC{cdot}mgChl-a{cdot}hr^{-1}$ and daily primary production ranged from 20.9 to 678.1 $mgC{cdot}m^{-2}{cdot}d^{-1}$(mean value was 206.72 $mgC{cdot}m^{-2}{cdot}^{-1}$).