The manufacturing by thermocompression of flax fiber composites requires a good understanding of process parameters. This work focuses on the influence of key parameters in the manufacture of quality unidirectional flax/polypropylene composites by thermocompression. To this effect, a number of matrix and process parameters have been studied and ranked according to their influence on composite morphology and tensile properties. Among tested parameters, process time and temperature tend to decrease tensile strength as they increased from 3 to 11 min and from 180 to 200 °C. As a result, decrease of cooling speed from 15 to 5 °C min−1 also decreased tensile strength. Temperature of 180 °C also led to composites with no dispersion of fibers. Using a compatibilized agent, maleic anhydride grafted polypropylene (PP), also increased tensile strength of composites. Increasing its percentage from 3 to 5% increased tensile properties in the 90° direction. Pressure had no significant effect on mechanical properties, though lower pressures increase fiber dispersion.