Rubber is used in many industrial applications, specially those need high damping capability. In present work, the effects of cyclic loading conditions on fatigue lifetime of the styrene butadiene (SBR) and nitrile-butadiene rubber (NBR) with 0,20,30,50 and 70 Phr of carbon black contents were investigated. Two cyclic loading conditions, namely full relaxation and non-relaxation tension, were considered. It was found that, in full relaxation tension test, i.e. R=0, fatigue lifetime increased with the decrease of the maximum strain. The decrease of maximum strain was accompanied with decrease in the maximum stress and stress amplitude. It was also found that, in non-relaxation tension test i.e. R>0, fatigue lifetime increased with the increase of the minimum strain, as well as, with the decrease of the stress amplitude. In this case, fatigue lifetime increased in spite of the fact that the increase of the minimum strain was accompanied with increase in the maximum stress at constant maximum strain. Consequently, it was found that the increase of minimum strain was beneficial in increasing the lifetime. It was also found that fatigue lifetime of rubber could be improved by increasing the carbon black content at the test conditions used in experiments.