The present experiments were conducted under laboratory conditions to study the effect of different constant temperatures (25, 30 and 35⁰C) on the development and mortality of the different stages of tomato leafminer, Tuta absoluta (Meyrick) , and to determine the thermal units required for T.absoluta  development in relation with degree-days (DD's).The incubation period lasted 5.0 ± 0.6, 4.0 ± 0.6 and 3 ± 0.6 days for eggs at constant temperatures 25, 30 and 35 ^oC, respectively. The duration of the larval stage was 18.1± 0.2, 12± 0.2 and 8.5± 0.2 days at 25, 30 and 35 ^oC, respectively.  The pupal average durations were 8.5± 3.1, 5.8± 1.4 and 5.2 ± 1.4 days at 25, 30 and 35^oC, respectively. The total developmental periods from egg to adult emergence were 31.6 ±4.5, 21.9 ±1.8 and 16.7 ±1.2 days for 25,30 and 35 ^oC, respectively.  These results proved that all the developmental periods decreased with increasing temperature. The thresholds of development (t₀) were 10.4, 16.3, 8.6 and 13.8 for eggs, larvae, pupae and all immature stages, respectively. The mean thermal constant required for development of T. absoluta eggs, larvae and pupal stages was 75.5 DD's, 160.8 and 182.3 degree-days, respectively