This experiment was conducted to investigate the development of rumen and its microbiocoenosis of lambs at 1 to 35 days of age under different milk replacing conditions. Fifty six Small Tail Han lambs with the birth body weight of (3 380±328) g were selected and fed milk powder substitute (MS, 28 lambs) and fish meal substitute (FS, 28 lambs) since 1 day of age, respectively. At 21 days of age, eight lambs from each of the two milk substitute treatments were turned to feed a starter. At the age of 7, 14, 21, 28 and 35 days, four lambs were sacrificed, respectively, to determine the differential count of the bacteria and protozoa, the number of fungus, the concentrations of volatile fatty acid (VFA) and ammonia nitrogen (NH₃-N) in rumen fluid, and the weight and morphological structure indices of rumen-reticulum. The results showed as follows: 1) there were protozoa (1.1×10⁵ cells/mL) and bacteria (85.47×10⁹ cells/mL, 97.4% Ruminococcus) in rumen fluid of lambs fed MS at the age of 7 days , and the total number increased with aging. There was a little of fungus in rumen fluid at 7 days of age, and the number became more until 35 days of age. Total VFA (mainly as acetic acid) was already produced in rumen of lambs at the age of 7 days, and the concentration increased obviously after the age of 21 days. The NH₃-N concentration in rumen fluid of lamb at 7 days of ages was 33.2 mg/dL. The weight of rumen-reticulum, and the height and width of rumen papilla of lambs fed MS increased with aging. 2) Compared with MS, the total numbers of protozoa and bacteria in rumen fluid of lambs fed FS were decreased by 30.8% (P<0.01) and 27.3% (P>0.05), respectively, the total VFA and NH₃-N concentrations in rumen fluid were changed dramatically with aging, the weight of rumen-reticulum was decreased by 17.1% at 35 days of age (P>0.05), the rumen papilla height was decreased by 16.0% (P<0.05), and the rumen papilla width was increased by 39.4% (P>0.05). 3) When the lambs were fed the starter instead of milk substitute at 21 days of age, the total number of bacteria in rumen fluid of lambs previously fed MS was increased by 49.7% (P<0.01), the total number of protozoa was decreased by 50.3% (P<0.01), the total VFA concentration was increased by 158.8% (P<0.01), and the weight of rumen-reticulum and the rumen papilla width were increased by 14.0% (P>0.05) and 61.8%(P<0.01), respectively, however, the rumen papilla height was shortened by 43.3% (P<0.01); the fungi number in rumen fluid was increased largely at 35 days of age. 4) When the lambs were fed the starter instead of milk substitute at 21 days of age, the total number of bacteria in rumen fluid was increased by 21.1% (P>0.05), the total number of protozoa was decreased by 60.4% (P<0.01), the total VFA and NH₃-N concentrations were decreased by 43.5% (P>0.05) and 49.1%(P<0.05), respectively, and the height and the width of rumen papilla were shortened by 23.1% (P<0.05) and 11.2% (P>0.05), respectively; there was little fungi in rumen fluid of lambs at 35 days of age. In conclusion, the development of rumen digestion of lambs starts at 7 days of age; feeding FS and starter both affects stable development of rumen and its microbiocoenosis.