This study investigated the impact of constant temperature(30±1ºC) on the developmental stages of the fall armyworm (FAW), Spodoptera frugiperda, and explored cannibalism behavior during the larval phase. The results revealed that FAW female exhibited an average fecundity of 696 eggs, ranging from 494 to 984 eggs during their entire lifespan. The pre-oviposition and oviposition periods averaged 2.66 and 1.33 days, respectively. Male and female longevity were 9 and 10 days, respectively. The egg incubation period was 1.35 days, with a hatching rate of 83.3%. Larval development took an average of 13.5 days, with the highest mortality rate observed in the 3^rd instar (30%) and the lowest in the 6^th instar (3.3%). Pupal development lasted 7.4 days, with an average pupal weight of 0.167 g and a 3.3% mortality rate. The complete life cycle (from egg to egg) averaged 37 days. Cannibalism occurred across all larval stages of the same age, peaking in the 4^th larval instar on both maize (4.72±0.47) and castor leaves (1.75±0.25). The lowest cannibalism rate was observed in the 6^th larval instar. Cannibalism increased with reduced food and smaller container sizes. This study provides crucial insights into the biological aspects and cannibalistic behavior of S. frugiperda, aiding in the development of integrated pest management strategies against this invasive species in Egypt.