In this thesis, the fatigue life of short fiber thermoplastic composites was validated with the predicted results from multi-scale approach using representative volume element (RVE). RVE model predicted the effective material properties and strength of the random short fiber composites based on the failure of micro-mechanics in the composites. These results were experimentally verified by fatigue life and strength analysis using BOSCH PA66 Nylon resin and short glass fiber composites, and Plastic Omnium Chopped Glass fiber/Vinylester. Three different short fiber thermoplastics, based on fabrication method, were used to verify the predicted results: (i) Longitudinally injected specimens and (ii) Milled specimens with different milling angles of 0o, 30o and 90o. (iii) Chopped GF/VE specimens. Tests were done for multiple fiber volume fractions (pure resin, 15%, 25%, 35% and 50% w/w), and fiber orientations with varying stress ratio (R=1, R=0.5 and R=-1) under the effect of temperature at room temperature, 80oC and 130oC. Fatigue tension-tension tests were carried out at 1 Hz for different load levels. Observed experimental results were compared with predicted values from the multi-scale approach. This analysis advocates the application of short fiber thermo-plastic composites in automobile, aviation and other industries as well as minimiz-ing the production time by predicting the material properties of the composite us-ing the aforementioned approach.