Determination of air-sea CO2 transfer velocity and flux is a key to constrain the global carbon fluxes. A widely used method to estimate air-sea CO2 flux has been based on so-called “bulk” CO2 method. This method requires the determination of the difference between air and sea surface CO2 partial pressure and the gas transfer velocity. The latter is typically obtained through the empirical relationship between transfer velocity and wind speed. Such a simplification would potentially induce uncertainties in the flux estimation due to for instance, the lack of on site gas transfer velocity data. Eddy covariance method is a direct measurement method, which in theory does not require any empirical parameters. During the past decade, significant progress has been achieved in the application of eddy covariance method in measuring air-sea CO2 fluxes. This paper is attempting to provide an overview and a comparison between various techniques available for the measurement of air-sea CO2 transfer velocity and for the estimates of fluxes. Principle, merit and demerit of each method will be briefly introduced. Our special attention is given to the eddy covariance technique, one of the potentially promising micrometeorological method.