Nine new 8-O-4' neolignans, named pinnatifidanin B I-IX (1-9), together with 9 known analogs (10-18) were isolated from the seeds of Crataegus pinnatifida. The structures of 1-18 were determined by spectroscopic methods, including 1D, 2D NMR, CD and HRESIMS analysis. Compounds 8-11, 17 and 18 displayed potent cytotoxic activities against human cancer cell lines, and most interestingly, none of the 6 compounds displayed inhibitory activity against human lung cell line (Mrc5). The 6 cytotoxic compounds are considered to be potential as antitumor agents, which could significantly inhibit the cancer cell growth in a dose-dependent manner and are probably safer than positive control drug.