In this paper, an improved seventh-order WENO (WENO-Z7) scheme is suggested by extending the 5th-order WENO scheme of Borges et al[R. Borges, M. Carmona, B. Costa, W. S. Don, An improved weighted essentially non-oscillatory scheme for hyperbolic conservation laws, J. Comput. Phys. 227(2008) 3191-3211]. The sufficient condition for seventh-order accuracy is described for the new smoothness indicator. The role of the parameter ε, which is used to construct the weights of WENO schemes to prevent the denominator from being zero, is discussed, and an optimized value of ε is suggested to improve the convergence and accuracy for practical applications.