A novel, simple and efficient method to prepare a new kind of nanocapsules in a demulsifying W/O micro-emulsion system from low molecular weight (MW) alginate and oligochitosan (OCS) was reported. Low MW alginate (LMWALG) was prepared by radiation of UV in the presence of cathode ion, OCS were used as anode ions. The composition for LMWALG demulsifying W/O micro-emulsion system is 50.75% (wt) of cyclohexane, 17.67% (Triton X-100 + n-pentanol), and 31.58% of LMWALG gelatum, while that for OCS one is 48.21% of cyclohexane, 16.09% (Triton X-100 + n-pentanol), and 35.70% OCS aqueous solution. The preparation of nanocapsules was optimized based on the above compositions for the MW of alginate, the MW of OCS, concentration of OCS and the encapsulation time. Ultimately the MW of LMWALG is 0.13 million and that of OCS is 3000 with a concentration of 1%, encapsulating for 30 min, under the above optimal conditions nanocapsules were formed in diameter of 136 nm.