Investigation of the physical properties on an n- type GaP single crystal semiconductor compound is given. The magneto-optical properties, magneto photoconductivity, Hall effect as well as Seebeck and Nernst effects are measured for the same sample. A constructed basic experimental set-up for conducting these measurements is shown. An optical cryostat which is capable to control the sample temperature in the range of (200-340 K) is shown. The cryostat is also suitable for measuring electrical as well as thermal measurements. Results of energy gap is obtained from both optical absorption method and photoconductivity measurements for zero and under the influence of a magnetic field .Reduced effective mass from both the magneto-optical and magneto-photoconductivity technique is given and explained. From the combination between electrical and thermal experiments, the method of four coefficients (Conductivity, Hall effect, Seebeck and Nernst) is explained as a direct measurement for both the density of state effective mass and the scattering parameter.