This paper considers the initial boundary value problem of solutions for a class of sixth order 1-D nonlinear wave equations. We discuss the probabilities of the existence and nonexistence of global solutions and give some sufficient conditions for the global and non-global existence of solutions at three different initial energy levels, i.e., sub-critical level, critical level and sup-critical level. 1. Introduction. In this paper, we consider the initial boundary value problem (IBVP) for the following 1-D nonlinear wave equation of sixth order u tt − au xx + u xxxx + u xxxxtt + f (u x) x = 0, (x, t) ∈ (0, 1) × (0, ∞), (1) u(x, 0) = u 0 (x), u t (x, 0) = u 1 (x), x ∈ [0, 1], (2)