The seedlings in greenhouse at a certain stage are transplanted from high density trays into low density flats for further growth and development. To meet the demands of transplant production in greenhouse, a simpletype automatic transplanter for plug seedlings was designed. The transplanter consisted of a manipulator, an endeffector and two conveyors. The manipulator consisting of a motor, a linear modules and a rodless cylinder, moved the endeffector to the desired working position for extracting, transferring and discharging a seedling. The conveyors were designed to move the plug tray/pot to the endeffectors working space. It was the pallettype doublerow chain transmission units. The endeffector was a pincettetype mechanism using two fingers and four pins for picking up seedlings. When the endeffector extracted seedlings from the tray cells, its two cylinder fingers push out four pickup pins to penetrate into the root soil deeply and then close making the pickup pins hold the soil of the seedlings firmly for lifting. When the endeffector released seedlings at the discharge point, its two cylinder fingers open making the pickup pins loosen the soil of the seedlings and then pulled the pickup pins back for discharging. According to the requirement of automatic transplanting of greenhouse seedlings, the electric control system was configured for the prototype. The step transmission for picking up seedlings was examined with the linear displacement sensor. The test results indicated that the transplanting rate was about 1221 seedlings per hour for the 128cell tray and 1025 seedlings per hour for the 72cell tray, respectively. The statistical analysis of onesample t test showed that there was no significant difference in the step transmissions between the testing value and the theoretical value, and the standard deviation was less than 0.5. The performance tests were further conducted to evaluate the machine efficiency. For the four locally produced vegetable seedlings, the average success ratio in picking up seedlings was up to 90.70%, and the soil damage ratio was less than 5%. The performance of transplanting was satisfactory.