This research successfully created an ultra-small orifice utilizing undercut fabrication process in a droplet-based microfluidics chip. The proposed novel T-junction structure with ultra-small orifice has a lot of advantages, including long-term stability for uniform droplets formation, reproducible ultra-small size droplet and tunable droplet size. The hydraulic diameter of the orifice is under 2 £gm, and the size of micro droplet produced from the orifice can be tuned to less than 10 £gm in diameter. Chitosan droplet can be produced by the proposed chip, which is usually adopted for medical applications. Surface modification technique was applied to modify the surface of microchannel to be hydrophobic for eaily producing hydro-droplets. Experimental results show that the ultra-small orifice microfluidics chip can steadily produce water-in-oil droplets only by controlling the flow ratio between dispersed phase and continuous phase flow rates. The size of the water-in-oil droplets can be tunable from 22 £gm to 6.5 £gm in diameter by adjusting the flow rate ratio of the continuous and disperse phase flows from 1 to 3.5 and the hydraulic diameter of the orifice is 1.1 £gm. And the size of the chitosan-in-oil droplets also can be tunable from 59 £gm to 27 £gm by adjusting the flow rate ratio of the continuous and disperse phase flows from 4 to 8. The proposed microchip has advantages including ease of control, low cost, and high throughput. The proposed technique can be widely applied on emulsion and micro droplet generation.