Lavender (Lavandula angustifolia L.) is the most important aromatic and medicinal plant in the world grown for its essential oil predominantly extracted from its aromatic leaves and inflorescence. The experiment was conducted with irrigation supplement during the 2017, and 2018 cropping seasons at the Wondo Genet Agricultural Research Center experimental field in Southern Ethiopia. One of the study's goals was to investigate the ideal plant spacing and harvesting age for lavender growth, yield, and yield components under supplemental irrigation circumstances. Three levels of harvesting ages following transplant (9, 10, and 11 months after transplanting (MAT)) and four levels of intra-row spacing (45, 60, 75, and 90 cm) with an inter-row spacing of 60 cm made up the experiment. Three replications were used in the factorial arrangement in the Randomized Complete Block Design (RCBD) experiment. The combined analysis's findings showed that the number of branches per plant, fresh leaves and inflorescence weight/plant, fresh leaves and inflorescence yield/ha, and essential oil yield/ha were all significantly impacted by plant spacing. Conversely, it had no discernible impact on the amount of essential oil or plant height. Plant height, fresh leaf, and inflorescence weight/plant, inflorescence yield/ha, and essential oil yield/ha were significantly impacted by harvesting age, but not the number of branches/plants. Fresh leaf and inflorescence yield/ha and essential oil yield/ha were significantly influenced by the interaction of plant spacing and harvesting age. The highest essential oil yield/ha (98.72 kg) and fresh leaf and inflorescence yield/ha (24.56 t) were achieved at 10 MAT and 60*45 cm spacing. Conversely, the 60*90 cm spacing and 11 MAT had the lowest essential oil yield/ha (34.42 kg). Therefore, it is highly recommended to grow Lavender with a supplement of irrigation at Wondo Genet and a place that has the same agroecology with a spacing of 60*45 cm and harvest it at 10 MAT to get the highest essential oil yield/ha.