The aim of this study was to investigate the effect of dietary supplementation of Thymbra spicata L. var spicata (TS) essential oil (10 or 15 μl · kg⁻¹ live weight (LW)) on growth performance (LW, LW gain, feed intake) and rumen parameters (volatile fatty acids (VFA)) and ammonia levels, and pH) in lambs. The experiment lasted 56 days, including 14 days of adaptation and 42 of experimental period. The TS essential oil was extracted from the leaves harvested at the beginning of plant flowering and contained: 66.86% carvacrol, 12.18% p-cymene, 10.73% γ-terpinene and 2.77% thymol. The addition of TS essential oil to concentrate feed did not affect final LW, LW gain, feed intake or feed conversion ratio (P > 0.05). Molar concentrations (mmol · l⁻¹) of total VFA and acetic (A), butyric (B) and propionic (P) acids, and levels of pH and ammonia-nitrogen (NH₃-N) in rumen fluid were also not affected by TS essential oil inclusion (P > 0.05). However, the proportion of A and (A+B):P ratio in rumen fluid VFA increased linearly (P < 0.01) with increasing TS essential oil doses, whereas the P proportion decreased linearly (P = 0.008). So, it could be suggested that TS essential oil enhanced the concentration of selected VFA in rumen fluid through a positive effect on feed digestion in the rumen. In particular, the linear increase in A and (A+B):P ratio could have enhanced fibrolytic effect in the rumen ecosystem. It was concluded that different doses and longer terms of use of TS essential oil should be further investigated in in vivo studies.