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We present an initial performance evaluation of InfiniBand HCAs (host channel adapters) from Mellanox with PCI Express interfaces. We compare the performance with HCAs using PCI-X interfaces. Our results show that InfiniBand HCAs with PCI Express can achieve significant performance benefits. Compared with HCAs using 64 bit/133 MHz PCI-X interfaces, they can achieve 20%-30% lower latency for small messages. The small message latency achieved with PCI Express is around 3.8 μs, compared with the 5.0 μs with PCI-X. For large messages, HCAs with PCI Express using a single port can deliver unidirectional bandwidth up to 968 MB/s and bidirectional bandwidth up to 1916 MB/s, which are, respectively, 1.24 and 2.02 times the peak bandwidths achieved by HCAs with PCI-X. When both the ports of the HCAs are activated, HCAs with PCI Express can deliver a peak unidirectional bandwidth of 1486 MB/s and aggregate bidirectional bandwidth up to 2729 MB/s, which are 1.93 and 2.88 times the peak bandwidths obtained using HCAs with PCI-X. PCI Express also improves performance at the MPI level. A latency of 4.6 μs with PCI Express is achieved for small messages. And for large messages, unidirectional bandwidth of 1497 MB/s and bidirectional bandwidth of 2724 MB/s are observed.