An influence of shear stress upon a thrust of conical nozzle with throat diameter of 10[-5]-10[-6] m has been under the study. Including of its input into determination of thrust was done with the use of the model of laminar boundary layer. It allowed to establish an existence of optimal nozzle length. This length was shown to depend essentially on throat diameter. An influence of properties of issuing gas and nozzle half-angle upon the thrust was studied. Developed method of estimation of nozzle thrust effectiveness can be used on preliminary step of design.