In this work we study the effect of LiNO3 on the Li anode and sulfur cathode, respectively, of Li/S battery by using a Li/Li symmetric cell and a liquid Li/Li2S9 cell. On the Li anode, LiNO3 participates in the formation of a stable passivation film, and the resulting passivation film grows infinitely with the consumption of LiNO3. The passivation film formed with LiNO3 is known to effectively suppress the redox shuttle of the dissolved lithium polysulfides on Li anode. On the cathode, LiNO3 undergoes a large and irreversible reduction starting at 1.6V in the first discharge, and the irreversible reduction disappears in the subsequent cycles. Moreover, the insoluble reduction products of LiNO3 on the cathode adversely affect the redox reversibility of sulfur cathode. These results indicate that both the Li anode and sulfur cathode consume LiNO3, and that the best benefit of LiNO3 to Li/S battery occurs at the potentials higher than 1.6V. By limiting the irreversible reduction of LiNO3 on the cathode, we have shown that the Li/S cell with a 0.2m LiNO3 as the co-salt can provide a stable capacity of ∼500mAhg−1.