A Computational Fluid Dynamics (CFD) based Numerical Wave Tank (NWT) is developed and verified to study wave load effects on fixed and floating offshore structures. The model is based on solving Navier-Stokes equations on a structured grid, level set method for tracking the free surface, and an immersed boundary method for studying wave-structure interaction. This paper deals with establishing and verifying a CFD based NWT. Various concerns that arise during this establishment are discussed, namely effects of wave reflection which might affect the structure response, damping of waves in downstream, and three dimensional effects of the waves. A method is described and verified to predict the time when incoming waves from wave generator are affected by reflecting waves from the structure which can help in better designing the dimensions of NWT. The model is then used to study sway, heave and roll responses of a floating barge which is non-uniform in density and limited in sway by a spring and damper. Also, it is used to study wave loads on a fixed, large diameter, surface piercing cylinder. The numerical results are compared with the experimental and other numerical results, and in general good agreement is observed in all range of studied wave frequencies. It is shown that for the studied cylinder, the Morison equation leads to promising results for wavelength to diameter ratio larger than 2pi (kD < 1), while for shorter wavelengths results in considerable over prediction of wave loads, due to simplification of wave diffraction effects.