The aim of this research was to investigate the effects of essential oils XTRACTTM and natural zeolite on blood cells morphology. The changes in the appearance of cells and nucleus of erythrocytes, lymphocytes and monocytes were measured in the control and experimental group, on the 21st and on the 42nd day of fattening. Image analysis was performed using the SFORM software (VAMSTEC, Zagreb, Croatia). A total of 36 blood smears were analyzed. Statistical data analysis was performed by use of the Statistica, 12. On the 21st day the size of erythrocytes was significantly lower compared to the control group, but on the 42nd day values returned back to usual. The changes in lymphocytes shape and size were seen on the 21st day of fattening. Their increased size and irregular shape remained the same on the 42nd day. The size of monocytes did not change until the 42nd day, after which they were significantly larger compared to the control group. Results of analysis of morphometric characteristics of erythrocytes, leukocytes and monocytes confirmed its importance in evaluation of the blood cells’ adaptive responses to stress factors during intensive fattening. The results show morphological characteristics of cell’s transformation in all examined variables of monocytes and lymphocytes as well as their nuclei: area, outline, minimal and maximal radius, convexity of area, length and breadth, whereby immunomodulatory effects of essential oils added in chicken’s food are confirmed. The obtained results can be used to promote effective methods for assessment of immune competence of the organism so far as the diagnosis and prevention of stress condition in intensive chicken fattening.