"Reasonable period of time"arbitration is one of three means of deciding implementation period under DSU. The primary responsibility of the arbitrator is to decide implementation time upon the request of the members concerned and, generally, does not involve implementation method and path. In practice, the burden of proof of this arbitration experiences a process from common proof to burden of proof being born by responsible party and domestic mandatory law has the prama facie evi- dence effect."Reasonable period of time"arbitration should be the possible shortest period to imple- ment the recommendations and rulings of the DSB. In addition,factors which effect on"the reasona- ble period of time"include the related domestic laws and regulations, implementation methods, com- plex degree, the real difficulty of developing countries and so on, which should be considered by arbi- trators.