High viscosity and low mobility are the main problems in transporting heavy crude oils through pipelines. The methods which have been used so far to overcome the mentioned problems include heating, upgrading, dilution, core annular flow, and emulsification in water. In this research, the emulsification method is thoroughly evaluated by investigating the rheological properties of crude oil-in-water emulsions. The factors affecting the rheology of a heavy crude oil-in-water emulsion stabilized by sodium carbonate (Na2CO3) as an actuator of natural surfactant (asphaltene) were studied. An Iranian heavy crude oil, namely West-Paydar, was used as the oil phase of the emulsions. The Taguchi method of an L-18 orthogonal array was employed to design the experiments. The viscosity of crude oil was almost Newtonian and about 0.3-11 (Pa.s) in temperatures ranging from 55 to 5 degrees C, respectively; while that of the emulsions was non-Newtonian and about one-tenth of the used crude oil. The rheological model of Herschel-Bulkley was fitted to the data of both emulsions and heavy crude. A purely mathematical calculation was used to determine the pressure loss per unit length of an assumed pipe. The pressure loss of O/W emulsion was about 90-99% less than that of heavy crude oil. The Herschel-Bulkley model was capable not only to predict the pressure loss at each particular condition, but also to optimize the process parameters according to pressure gradient, temperature, volume fraction, sodium carbonate content, and salinity in a manner to be more close to natural conditions as well as economic terms.