This investigation was darned out to study the influence of differ‌ent temperature degrees, short ultra -violet and direct sun light on the rate of decomposition of pirimphos - methyl, fenitrothion ,malathion and prothiofos insecticides, which are widely used in Egypt to protect vege‌table crops from economic pests. A. Stability of pirimphos-methyl, fenitrothion, malathion and prothiofos insecticides under different temperature degrees . The results indicated that the increase of temperature degrees means an increase in insecticide residue loss_ the percentage loss of pi‌rimphos-methyl, fenitrothion, malathion and prothiofos insecticides was 8.69%,1.33%, 2.42% and 0.57% at20°C, 41.70% , 23.13%, 20.23% and12.86% at 30°C ;97.56%, 60.28% ,73.56, and 55.81% at 40°C and 99.98%, 97.38%, 93.855% and 89.59% at 50°C, respectively after 192 hours of exposure. On the other hand, the half life values of these insecticides were 1463.68, 9918.52, 5432.99 and 23243.03 hours at 20°C ; 246.68, 506.08, 587.26 and 966.71 hours at 30°C; 61.51, 134.67, 123.22 and 162.97 hours at 40°C; and 18.45, 40.47, 56_49 and 93_18 hours at 50°C, respectively. In general, icreasing temperature degrees and prolongation of ex‌posure increase the percentage loss of insecticide. Prothiofos insecticide decomposed less than the three others. Fenitrothion, and malathion showed moderate thermal degradation, pirimphos-methyl had the high‌est thermal decomposition during the periods of experiment_ B.Ultra violet ray effect. The obtained results showed that fenitrothion, malathion and prothiofos residues were more stable than pirimphos-methyl when ex‌posed to UV-rays. The percentage loss of pirimphos-methyl, fenitrothi‌on, malathion, and prothiofos insecticides after 12 hours were 78.97%, 39.46%, 42.47% and 48.30% respectively. The half life values of these insecticides were 6.41, 14_96, 13.55, and 12.61 hours, respectively. The rate of degradation of the four tested insecticides varied ac‌cording to their chemical structure, time of exposure and the wavele‌ngth of UV-rays used. Pirimphos-methyl was the most affected by UV ir-radiation among the four insecticides.