The laminar boundary layer theory is used to analyze the film condensation on a vertical plate. The calculated result is well consistent with the experiment and empirical relation of the laminar film condensation. A convection number E is presented

which shows the contribution of convection to heat-transfer in film condensation. When E is less than 1

the condensation region is controlled by conduction. When E is far less than 1

the region is called Nusselt region. When E is larger than 1

the region of condensation is controlled by convection. When E is much more than 1

Co_m is proportional to Re_L~1/3.