Purpose. To investigate the content of 2-thiobarbituric acid reactive substances (TBARS) in the walnuts oil with the use of rosemary essential oil (Etja, Elblag, Poland) as an antioxidant agent by monitoring the lipid peroxidation during 40 days storage period. Methodology. Lipid oxidation was evaluated by TBARS according to the method described by Kamyshnikov with some modifications. All variables were tested for normal distribution using the Kolmogorov-Smirnov test (P>0.05). Significance of differences in the lipid peroxidation biomarker in the samples (significance level at p<0.05) was examined using the Mann-Whitney test according to Zar. All statistical calculations were performed on separate data from each sample with STATISTICA 8.0 software (StatSoft, Krakow, Poland). Scientific novelty. Investigated the content of 2-thiobarbituric acid reactive substances (TBARS) in the walnuts oil with the use of rosemary essential oil Etja, Elblag, Poland. Conclusions. This study evaluated the effects of rosemary essential oil (REO) as an antioxidant agent by monitoring the lipid peroxidation in the walnuts oil during 40 days storage period by the assessment of the content of TBARS as a biomarker of lipid peroxidation. The effect of the REO on oxidative stability of the walnuts oil was evaluated throughout 40 days of storage. The inclusion of the REO in plant oil and storage time significantly affected TBARS values at 8 days. Lipid oxidation decreased significantly (p<0.05) during storage, particularly in the control sample, which showed the highest decrease at 15 days (by 57.14 %, p<0.05) and at 8 days (by 3.6 %, p>0.05). The REO decreased lipid oxidation compared to the control sample by 13.6% (p<0.05) at 8 days and increase by 10.9 % (p>0.05) at 40 days. At 15 days, the TBARS values reached approximately 73.85 μmols × L-1, corresponding to a lipid oxidation increase of approximately 19 % (p>0.05) for samples enriched by REO. The present results demonstrate that the administration of REO, exhibiting free radical scavenging activity determined by TBARS assay, exerts beneficial effects on preventing lipid peroxidation in walnuts oil by limiting the TBARS level at 8 days of storage.