The mechanical properties and hydrogen embrittlement behaviors (HE) of Zr-4 alloy in the temperature range of 40-4.2 K have been investigated. The hydrogen free Zr-4 alloy sample exhibits a very good tensile strength and superior ductility as high as a 21&percent; increase at liquid hydrogen temperature (20 K) over that at room temperature. It also shows the typical features that appeared in austenitic stainless steel, such as unstable plastic flow, strengthening and multi-necking behaviors. The difference is that the temperature of these behaviors occurred much lower than those in austenitic stainless steel. The experiment on hydrogen charged sample shows a heavy hydrogen embrittlement behavior at low temperatures. The hydride cracking mechanism of HE has been discussed in the paper. ©2002 American Institute of Physics.