Field experiments were conducted at four locations in farmer fields at Vadivellampalayam, Panaiyampalli and Pungampalli villages in Tamil Nadu during rabi season (2012-13) to establish the optimum dose of Cu for onion crop. The experiment was laid out in a randomised block design replicated thrice with seven levels of Cu application viz., 0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 kg ha-1 along with recommended fertiliser dosage 60:60:30 kg N, P2O5 and K2O kg ha-1. The results revealed that the graded level of copper application significantly influenced the growth (plant height and No of leaves per plant) and yield parameters viz., fresh of total plant, bulb and leaf, dry weight of bulb and leaf, number bulbs per plant, bulb diameter. The application of 1.5 kg Cu along 100 % RDF of NPK @ 60:60:30 kg ha-1(T4) registered the highest fresh bulb yield of onion at harvest stage, ranging from 20.5 to 22.1 t ha-1 and it was 32 to 43 % increased over control than other treatments. The response of onion plant to copper application, bulb yield increased with increasing doses of copper up to 1.5 kg Cu ha-1 and beyond this excess application of Cu decreased the bulb yield. The same treatment proved its superiority by recording 35 to 44.9 % and 30.9 to 47.3% increased total fresh plant yield during bulb initiation and harvest stage. The various available Cu status soils, the positive response to applied Cu nutrients were noticed until the DTPA Cu status of 0.62 mg kg-1 (location 3) and thereafter the increasing amount of DTPA- Cu showed the negative response.