Harvest time, in olive, is an important key to achieve high quality and quantity of oil with a desired and balanced fatty acid (high oleic acid and low palmitic and linoleic acid) composition. Appropriate harvesting time varies in different locations depending on climatic and agronomic situations and identifying the right time for harvesting may bring about a high quality oil. Thus, the current study was carried out to determine the best harvesting time for two Iranian (Mari and Shenge) and two foreign (Kroniki and Arbequina) cultivars in the region of Tarom, Zanjan, (north of Iran). The results obtained indicated that palmitic, oleic and stearic acid contents decreased and linoleic and palmitic acid contents increased with the progress of ripening. In addition, oil percentage increased with the progress of fruit growth and development. Based on the amount of desired fatty acids, the best harvesting time for the Mari, Kroniki and Arbeqina cultivars, was 180 days after full bloom. It was revealed that the cultivar Shenge is not suitable for oil extraction, due to the low percentage of oleic acids. Therefore, Shenge could be cultivated for producing olive cans rather than olive oil and the best harvesting time for Shenge is 120 days after full bloom.