Cylindrical Spiral Mandrel Dies (SMD) and Flat Spiral Dies (FSD) are widely used in blown film co-extrusion processes. Although in recent years the use of stackable (flat) spiral dies in the blown film extrusion has been continuously growing, a phenomenological study on designing parameters of this type of dies, and comparison of their flow distribution with SMD has not been made yet. In this work, the flow of a viscous power law fluid in a typical flat spiral die, based on Rauwendaal's method (1987) for analyzing cylindrical spiral mandrel dies, has been investigated. Validation of this method for flat spiral dies was confirmed by comparison with the results reported using the finite element method. The effects of some design parameters, such as number of grooves, initial flight clearance, initial groove depth, on pressure gradient, flow distribution and other operational variables have also been studied. It has been found that the flow distribution becomes more uniform with increasing number of grooves, initial groove depth and coefficient of Archimedes type flat spiral. Moreover, the results have shown that the increment of the clearance angle between two die plates, as well as the initial clearance, have an optimum value in which the flow distribution is the most uniform. Finally, flow variations in FSD and SMD have been compared for different design parameters. The results demonstrated that the melt flow distribution at the outlet of the cylindrical spiral mandrel die is more uniform, while the pressure loss of the flat spiral die is lower.