Abstract: Abstract: A stable end-effector in transplanting seedlings is a key technology for an automated transplanter in mechanized agriculture. The optimization parameters for gripper needles clamping seedling plugs could provide information for end-effector design and seedling plug nursery, which would be more suitable for mechanical transplanting. A seedling plug clamping platform with adjustable needles mounted on a universal tester has been designed in this study to simulate the clamping action of the needles. The vertical compression force of the universal tester was converted to horizontal compression of needles for clamping plug by mechanisms of (a) a higher pair with bearing-slope and (b) a sliding pair with a linear guideway-slider. The mechanical transmission model of the feedback force system was established for obtaining the clamping force value from the universal tester data of plug clamping operation subtracted from operation without the plug. The horizontal compression deformation value corresponding to the plug compressive value was converted from the vertical deformation by the geometrical relationship of the mechanism. In total, 45 cucumber seedling plugs with 10 days' cultivation in a greenhouse were tested in this research. Five parameters related to the needle and plug components were tested separately to demonstrate the effect of clamping force. They were four clamping needle angles with verticality (4°, 7°, 10° and 13°), three and four gripper needles, four sets of moisture content plugs (65%, 75%, 85% and 88%), three stages of seedling growth, and two kinds of bulk material volume proportion. Clamping force values were reliable which gripper needles got at clamping needle angle of 7° and 10° than that at 4° and 13° , especially the value was best at 7° than other three angles. Four clamping needles can obtain a higher clamping force value than three needles because of symmetrical compression with less external deformation. The clamping force can be changed by different moisture content with conversion of material adhesion stress and plasticity. A seedling plug with 84% moisture content got the best performance among the four levels of moisture content plugs. Seedling growth with a taproot length flourish improved the shear strength and structural stability of the plug. A plug with stem length or taproot length separately exceeding 30 mm and 87 mm had better clamping character than a plug exhibiting weaker growth. The volume proportion of bulk material which was made up of turf, roseite, and perlite with 6:3:1 had less compression space in the plug than bulk material with 7:2:1 because of the stiffness associated with increasing the roseite dose. Also, the former volume proportion plug got a bigger clamping force value with the same compression deformation. Methods of mechanical design and seedling cultivation could attain the optimal parameters verified in these tests. The end-effector with needle motion could obtain the stable, increased clamping force between the needles and seedling plugs, which will improve the grasping reliability in robotic transplanting.