Abstract: In the present paper the variational solution of velocity profile for an incompressible laminar and fully developed flow in isosceles triangular ducts is derived by applying the Kantovorich method. The theoretical and experimental results of pressure loss are also given. The velocity distribution model, additional pressure loss coefficient and calculating method of inlet length in the entrance region of isosceles triangular ducts are also derived, which are suitable for various kinds of vertex angles. The calculations and experiments are also performed for two models: the isosceles triangular channels with vertex angles 2a=45.1° and 2a= 60°. Comparisons are made between the theoretical analysis in this paper and those of the other authors. It can be seen that the present analytical result is of high. accuracy and wide practicability, and agrees well with the authors’ experiment.