Biological control of the two-spotted spider mite, Tetranychus urticae Koch in Behera Governorate on watermelon field and greenhouse are possible by using the predatory mite, Phytoseiulus persimilis (A.-H.). The predacious mite was released at three levels 20, 30 and 40 individuals/bit in open field, while was released in greenhouse at one level with 30 individuals/bit, the best one in the level C (40 individuals/bit) followed by B and A levels, respectively. The percent reduction of T. urticae population in open field reached 98.25% after seven weeks from released at level C (40 individu-als/bit), while reached 94.82 and 92.38% at levels B and A, respec-tively. The reduction percentage of T. urticae in greenhouse at rate 30 individuals/bit was 98.83% in the end of experiment. This re-sults indicated that the possibility of this predator to control T. urti-cae on watermelon in open field and greenhouse.