This study investigated the effects of various dietary forage-to-concentrate ratios (F: C) on urinary excretion of purine derivatives (PD) and microbial nitrogen (MN) yields from the rumen of Dorper ´ thin-tailed Han crossbred sheep. Twelve Dorper × thin-tailed Han crossbred ram lambs (47.2 ± 1.0 kg) fitted with ruminal and duodenal cannulas were randomly assigned to 12 levels of dietary F:C in a balanced incomplete block experimental design (12 lambs × 4 periods). Digestibility trials were conducted and MN was estimated using either ¹⁵N or PD as markers. Urinary excretion of allantoin and total PD increased (P < 0.05) with decreasing F:C. Urinary excretion of uric acid or xanthine plus hypoxanthine was unaffected by F:C (P > 0.05). MN yields estimated using ¹⁵N as a marker were greater than those predicted from urinary PD (12.5 vs. 11.5 g/d, P < 0.05), but the former was more variable than the latter (SE = 0.66 vs 0.45, respectively). A linear correlation existed between MN estimated by ¹⁵N and urinary excretion of PD: MN (g/d) = –0.521 + 1.493 × PD (mmol/d) (r² = 0.86). The purine nitrogen index (PNI: PD nitrogen/urinary N) was linearly correlated with nitrogen capture efficiency calculated from either ¹⁵N or PD (r² = 0.60 and 0.77, respectively). Results suggest that urinary PD is an accurate indicator of MN from the rumen of Dorper crossbred sheep, and PNI reflected the conversion of nitrogen degradation to MN in the rumen.