Fatigue crack propagation rate and threshold characteristics of the SA516/70 steel which is used for the low temperature pressure vessels, were studied in the room temperature of $25^{circ}C$ and low temperature ranges of $-10^{circ}C,;-30^{circ}C,;-60^{circ}C;and;-80^{circ}C$ with stress ratio of R=0.1. In the logarithmic relationship between the fatigue crack propagation rate($d{alpha}/dN$) and stress intensity factor range ${Delta}K$, the linear relationship was obtained up to $d{alpha}/dN=4.425{ imes}10^4mm/cycle$ in the same of room temperature, but in low temperature case, the relationship was extended to the range of low crack propagation rate. The fractured specimens were examined by SEM. Tested results showed that specimen failed at low temperature exhibit the quasi-cleavage fracture formation however considerable ductility proceed final fracture.