The effects of pistachio by-products aqua extract (PBE) on growth performance and retention of dietary nitrogen (N) were investigated in finishing lambs fed diets containing 11.3% DM (low protein; LP) or 13.9% DM (high protein; HP) crude protein. Two groups (n = 10) of Baluchi ram lambs (4-5 months old, 26.25±3.30 kg initial BW) were allotted to LP and HP treatments. In each group, one half of the lambs received experimental diets mixed with 500 ml liquid fresh PBE per kg DM of diet. Thus, four experimental diets were tested: LP diet without PBE supplementation; LP-PEB, LP diet with PBE supplementation; LP+PBE, HP diet without PBE supplementation; HP-PBE and HP diet with PBE supplementation; HP+PBE. Feed intake, average daily gain (ADG) and feed conversion ratio (FCR) were recorded for a 50 d experimental period. Plasma urea (PUN) and ruminal NH₃-N concentrations were measured at 0, 2 and 4 h after morning feeding on the 35^th d of the experiment. Apparent total tract digestibility of DM, organic matter (OM) and N as well as urinary losses and retention of N were also determined for 5 days at the end of the experiment. Although there was a tendency for feed intake to be lower (P=0.102) in LP compared with HP fed lambs, the dietary CP level had no effect on ADG and FCR. In contrast, PBE supplementation improved both growth rate (P=0.021) and FCR (P=0.104) of lambs. The urinary losses of N and also PUN and ruminal NH₃-N concentrations at h 2 and 4 after feeding were decreased and the retention of N was increased by PBE supplementation of diets. However, these effects of PBE were only significant in the lambs fed HP diet. The concentrations of PUN (0 and 4 h after feeding) and ruminal NH₃-N (4 h after feeding) were affected positively by dietary CP level. Although the lambs feeding on the LP diet retained a higher percentage of N intake or N digested, no difference in daily N retained was observed between LP and HP fed lambs. The experimental treatments had no effect on DM, OM and N apparent digestibility. Our results indicated that the low concentration of PBE tannins have a protein-sparing potential depend on dietary CP density, thereby improving growth performance and N retention in finishing lambs.