It was hypothesised that Nannochloropsis limnetica due to the specific chemical composition and fatty acids profile, may positively affect rumen fermentation. To confirm this hypothesis the batch culture experiment was conducted to evaluate N. limnetica supplemented at 0, 2, 4 and 6% of the substrate dry matter (DM) on both fermentation and fatty acid proportion in the ruminal culture. It was found that microalgae N. limnetica contain (g · kg⁻¹ DM): crude protein 238, Ca 48.7, Na 31.8 and unsaturated fatty acids (51.7 g · 100 g⁻¹ fatty acids). Moreover, leucine and lysine were the most abundant essential amino acids in the analysed microalgae. The total bacteria count was negatively affected if N. limnetica algae were supplemented at more than 4%. So, the research hypothesis that microalgae N. limnetica may affect rumen fermentation was confirmed, mainly by increasing propionic acid concentration without changes in the total volatile fatty acids concentration. However, a high dose (6%) decreased rumen bacteria count. Further research under commercial farm conditions should be conducted to confirm the usefulness of freshwater microalgae N. limnetica as a feed additive for ruminants.