We present an Investigation of continuous wave fiber laser (FL) doped with ytterbium Yb3+. This (FL) can be relied upon in many applications, due to Yb3+ physical properties that make it a unique laser with wide range of wavelengths (from 974nm to 980 nm and another from 1010nm to 1170 nm. Some of laser parameters were calculated using further order range-kutta method using MATHCAD V 14 program. The calculation includes found the main parameters of (FL) which are pumping power, signal power in positive and negative directions. Also other parameters that depend on the main parameters and represent the design parameters such as output power and gain and the optimum fiber length. Scattering effect for various parameters was studied. Also error was found between signals power in positive and negative directions. The study concluded that scattering is affecting the performance of all design parameters and the error in signal powers did not exceed four percent along the fiber length.