Numerical range and numerical radius play an important role in the fields of analysis, operator theory and mathematical physics. In recent years, there are many generalizations about classical numerical range and numerical radius, and many mathematicians have paid considerable attention to the study of these concepts. In this talk, we introduce some refinements and generalizations of existing numerical radius inequalities for Hilbert space operators. Further, we obtain some upper bounds of the numerical radius. We also obtain some numerical radius inequalities for 2x2 operator matrices which improve on some existing results.