Vertical distribution of a copepod Oithona davisae was modeled by an advection-dispersion model. The advection speed is expressed by two factors; the swimming speed due to biological geotaxis or barokinesis and downward sinking speed due to gravity. The vertical dispersion effect of the population is expressed by the dispersion coefficient. These numerical parameters were obtained from the swimming behavior of individuals and from the distribution of the population in an cylindrical experimental water column; 3m in hight, 0.19m in diameter. The swimming behavior of individual copepods was recorded by the video-camera or by visual observation. Anethetized individuals were entered into a glass test tube to measure sinking speed. Upward and downward directional components of the swimming speed were introduced in the model using the selecting probability in the case of steady state vertical distribution. In the experiment of the population distribution, sea water containing copepods was entered into the experimental cylinder. At the start of the experiment, the density of copepods at each developmental stages was vertically homogeneous. The distribution pattern sifted to steady state within 1 hour at all developmental stages. In the observation of individuals three modes were observed in the swimming of O. davisae. Those were defined to the 'rapid mode', the 'burst mode', and the 'fluttering mode'. Only the burst mode and the fluttering mode were observed when the distribution pattern reached the steady state. The obtained probability to select upward direction was 0.9 in the copepod swimming. The upward swimming speed almost balanced with the sinking speed and the distribution was very sensitive to the selecting probability. This suggests that copepods can easily change their distribution. This quantitative model is applicable to predict the vertical distribution in the sea with more information of the swimming behavior responsing to environmental conditions.