Abstract A flax fiber pilot plant is needed to process small samples of flax straw into fibers to facilitate research on retting and fiber properties. Our objective was to develop and test a modular design for a flax processing pilot plant based on a commercial line that was capable of cleaning fiber and seed flax straw from unretted, dew-retted, and enzyme-retted samples. The USDA Flax Fiber Pilot Plant (Flax-PP), which is the only research facility of this type in the United States, was designed according to the commercial ‘Unified Line’ (Czech Flax Machinery), but smaller and constructed in four individual modules. The modules and their order for processing were as follows: 9-roller calender, top shaker, scutching wheel, top shaker, 5-roller calender, and top shaker. Illustrations and diagrams of the operating modules are presented. Unretted ‘Neche’ linseed flax, dew-retted ‘Natasja’, and enzyme-retted ‘Jordan’ fiber flax were processed, and the cumulative weight loss of material at successive processing steps was determined to assess the effectiveness of cleaning. Fiber strength, fineness, and elongation were determined for the retted samples after cleaning through all the steps in the Flax-PP. A yield of fine fiber from the retted stems processed through the Flax-PP was acquired from further cleaning and refining by passage through a Shirley Analyzer. The various samples behaved differently at different stages of processing and the resulting fibers had different properties. The dew-retted Natasja fibers were stronger and finer than the enzyme-retted Jordan flax after pilot plant processing, but the Jordan fibers appeared cleaner and better retted. The Flax-PP effectively processed samples of diverse characteristics and will facilitate integrated research on retting methods for fibers with tailored properties.