It is very important to choose appropriate fiber parameters such as core and outer diameters and refractive index difference of fiber for obtaining low loss and stable fiber cables for practical use. Several problems related to determine the fiber parameters must be taken into concideration for a design of fiber structures. Many theoretical and experimental studies on the relationship between fiber parameters and transmission characteristics to show fiber parameters dependence on excess loss useful for the design of practical fiber cables. In this paper, we made clear experimentally the relationship between excess losses and fiber parameters for jacketed step-index fibers with various combinations of core and outer diameters, index difference and jacket diameter. As the results, we found that the excess loss caused by winding uniformly on the mandrel is proportional to (a/)2-3 and is independent of fiber diameter and that the loss due to microbending by external forces remarkably affected by both a/ and the fiber and jacket diameters. The results obtained by the investigations are utilised for manufacturing low loss and stable optical fiber cables.