A blade is usually mounted on the head of the onion harvester at certain angle to take the bulbs out of the soil. The blade angle and harvesting depth have significant effects on the efficiency of the harvester and also on the percentage of the damaged onion bulbs. In this study, a head for onion harvester was designed and developed and then the performance of the machine was investigated in a farm with clay-loam soil which the plants were planted in rows. A four bar mechanism was used for changing of the blade angle. The experiments were performed at vehicle speeds of 1.8, 2.4 and 3 km/h and the blade angles of 12, 15 and 20 degree. A factorial experiment was conducted based on a randomized complete design. The best performance of the harvester was found at vehicle speed of 1.8 km/h and the blade angle of 20 degree. The percentage of the damaged bulbs caused by the harvester was less than the manual method.