A new evaporimeter with flat leaf-shaped plates of both surfaces wetted was designed and calibrated. This equipment consists of a set of two this 5cm×5cm flat plates within which electrical resistor-sheets of uniform resistance are enclosed. One of the plates has the evaporating surface of filter-paper wetted and the other has the non-evaporating surface covered with thin polyethylene sheet. These two plates whose leading edge being in line, are set on a plane and thermal insulators hold them. Water is supplied to the evaporating surface from a small reservoir, homogeneously in the transverse direction along cotton thread through vinyl tubes.Electric power consumed in the non-evaporator is kept at a constant value, and caloric value caused by electrically heating the evaporator is adjusted so that the local surface-temperatures of the evaporator and non-evaporator at the identical position become to be equal. Then, from the difference between the equations of local heat-balance of both plates at a distance of xcm from the leading edge, neglecting heatconduction within the plates, under the condition of identical absorptivities to solar and infrared irradiations and heat capacities of the plates, the following equation is written;Lwx=Q-Q′, where, wx is local evaporation rate in g/cm2 sec, L; heat of vaporization of water (cal/g), Q and Q′; electric power consumed per unit area of evaporating and non-evaporating plates (cal/cm2 sec), respectively.Here, local evaporation rate of a plane surface can be obtained from the electric power compensating the latent heat of vaporization in evaporating surface.Coefficient of local transfer of water-vapor by forced convection, Df, x (cm/sec), is evaluated from the following equation;Df, x=Dx, EXP-Dn, CALC, where, Dx, EXP=wx/2ΔCx and ΔCx=CE, x-cA(g/cm3); difference in vapor concentration between the wet surface (CE, X) and the air outside the boundary layer on the surface (CA), Dn, CALL=0.405 (Gr'·Sc)0.25 d/l; calculated value of average coefficient of water vapor transfer from the upper and lower surfaces of the plate for free convection, Gr′; Grashof number in vapor transfer, Sc; Schmidt number, d; molecular diffusivity of water vapor to air (cm2/sec), l; dimension of the surface along the wind-direction (cm).As the results of some tests by means of a wind tunnel in which average intensity of turbulence was about 0.6%, the local water-vapor transfer coefficient at x=2cm was obtained as follows;in the range of Rex under 2×104, Df, x=1.15⋅λ⋅0.332Sc1/3⋅Rex0.5⋅d/x, where, λ is the correction factor for the effect of vapor concentration-departure distribution on evaporating surface, Rex; local Reynolds number.The above expression of Df, x showed the property of the transfer coefficient for laminar boundary layer.