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Processing flax fiber-high density polyethylene (HDPE) composites utilizes crop by-product (flax straw into flax fiber) in the plastic processing industry and benefit the environment. Injection molding is an important processing method with the characteristic rapid production rate. During injection molding, materials are melted which usually have non-Newtonian flow behavior inside the mold. Knowing the rheology of materials is beneficial in predicting injection process performance and melt flow. In this study, flax fibers were compounded with HDPE by twin-screw extruder and before they went into injection molding, their apparent viscosities were measured by a capillary rheometer at temperatures ranging from 170 to 200C. Experimental results showed that HDPE and flax fiber-HDPE biocomposites exhibited shear thinning behavior when they were melted; they had the characteristics of a power-law fluid during injection molding. Shear rate, injection temperature, and flax fiber content had influence on biocomposite viscosities. Biocomposite with a high viscosity is difficult to mold to a required shape. It was found that lower viscosity was observed from higher temperature, lower fiber content and higher shear rate. Linear and nonlinear regression models for shear stress and viscosity were developed using SAS statistical software.