Cognitive radio (CR) network allows fast deployment of wireless technologies to utilize spectrum channels, all with minimal impact on existing primary users. Another challenge in CR networks is the spectrum handoff issue when the primary user (PU) appears in the spectrum band being used by the secondary user (SU). In this paper, unlike the existing spectrum handoff schemes suitable for fixed channel bandwidth, we introduce the concept of the delay bandwidth product (DBP) to prioritize the channels with variable bandwidths. The delay in the proposed DBP index is defined as the difference of the maximum tolerable delay of the SU and the average occupation time of the PU. Based on the DBP index for the variable bandwidth channels, the SU selects the optimal channel and bandwidth that can deliver the highest throughput and guarantee its QoS requirement. Compared with other existing spectrum handoff schemes, the proposed DBP-based spectrum handoff can achieve 100% to 200% higher throughput subject to the delay requirements for supporting voice and web browsing services.