Abstract: To understand the anatomical development of whole digestive tract and functional development of the rumen in grazing lambs at the age of 056 d, fortyfour Gansu Merino male lambs were killed and sampled at 0, 3, 7, 14, 21, 28, 42, 56 days of age, respectively. The relative weights of gastric ventriculars(% live weight, RWL) of forestomach, abomasum, small intestine and cecum, relative weight(%of whole stomach weight, RWS) and volume(%of whole stomach volume, RCS) of stomach compartments, relative weight of the compartment contents(% of whole stomach contents weight, RWCS) and the pH of the compartment contents were measured; the concentrations of VFA and activity of microorganism enzyme in rumen content were also determined. With the age increasing, RWL of forestomach, small intestine and cecum increased with the faster increasing of forestomach and cecum at day 1421, followed day 4256, the increasing of small intestine slowly. RWS and RCS of rumen increased very markably, while these of abomasum reduced, all the changes were faster at day 714, 1421 and 4256. During 014 d, RCS was lower than RWS for forestomach, but thereafter the growing was faster for former than that for later, hence the value of RCS exceeded RWS after day 28. With the age increasing, the pH of contents in stomach compartments was stable relatively, the pH of rumen contents ranged from 6.03 to 6.67. 07 d the ratio of occurring rumen cellulase activities in partial detected lambs increased with the age increasing, after 14 d the enzyme activities had been observed in the all lambs tested, and the activities increased with the age increasing too; the peaks of activities occurred at day 14 for protease and at day 42 for amylase and cellulase. Acetate, propionate and butyrate were found from lambs′ rumen contents of day 3 and the concentration of acids raised with age increasing. The appearance of peaks of VFAs concentration and activities of the amylase and cellulase were parallel with the development of rumenreticulum. The faster growing during day 021 and thereafter the slower growing for RWS of forestomach was caused by the change of nutrient state of lambs. Under the study condition, lambs′ weaning is better at 2 monthold, and day 42 is the earliest weaning age accepted.