We propose a new size-based burst classification algorithm which assembles arriving packets with different burst size according to classes. in OBS ingress switches. In JET-based OBS scheme, because the high priority class bursts are likely to occupy channels in advance, the low priority class bursts should find void intervals between high-class bursts to avoid burst blocking. The research results show that there was close relation between blocking rate of low-class bursts and size of its bursts. By limiting the maximum length of lowclass bursts, loss, throughput and assembly delay performances are improved when proposed scheme is compared to conventional scheme.