Injection molded short natural fiber composites (NFC) are studied in this article. Micromechanical models are employed to investigate effects of fiber length distribution, fiber orientation, and fiber/matrix adhesion on mechanical performance of NFCs. A parametric study showed that length/diameter ratio strongly affects the composite strength, whereas fiber strength has very little influence on strength of the composites when fiber/matrix adhesion is poor. In order to verify conclusions based on theoretical predictions, modeling results are compared with experimental results of wood powder-, flax-, pulp-, and glass fiber composites. Recommendations on improvement of strength of NFC through change of the internal structure of the materials are formulated based on results of this investigation. © 2007 SAGE Publications.