The microbial colonization of the rumen after birth is influenced by several factors that may modulate the microbiota composition and function. We report here the effects of a treatment applied in early life on rumen fermentation and archaeal community structure in lambs from birth up to 4 weeks of age. Sixty lambs and their dams were allocated to two groups. Treatment (T) consisted in linseed oil and garlic essential oil drenched daily during the whole experimental period. T was also applied to dams. Rumen contents were sampled by oral stomach tubing. All lambs were sampled at 1 week, as well as all dams for comparison. A representative set of lambs (n=10) was sampled at 2, 3 and 4 weeks. Volatile fatty acids (VFA) were analysed and 16S rDNA amplicon sequencing was used to explore the archaeal diversity. Total VFA concentration was not affected by the treatment except in 1-week-old lambs (34.4 vs 24.3 mM, for C and T, respectively, P<0.05); VFA profile was mostly affected in T dams with lower proportion of acetic (64.5 vs 71.1%, P<0.001) and higher proportion of propionic acid (20.6 vs 13.8%, P<0.001). In lambs, archaeal diversity increased with age but changes were less marked in T animals; Shannon indices were lower in T 4 week-old lambs and ewes (P<0.001) and numerically lower in T younger lambs. Principal coordinates analysis were constructed independently for each age group using the weighted UniFrac similarity matrix and differentiation between T and C animals increased as age increased. These results indicate that the treatment changed the development of a conventional archaeal rumen community in young lambs without affecting VFA concentration in rumen.