This study investigates factors affecting group sizes of Maasai giraffe (Giraffa camelopardalis tippelskirchi) in the Serengeti-Ngorongoro ecosystem, northern Tanzania. Various studies on giraffes have been conducted in the past, but none has investigated factors affecting giraffes' group size and vigilance behavior in this vast ecosystem. As a result, I was motivated to investigate the following hypotheses: 1) in a given group, the sex ratio of males to females with calves is related to group size; 2) group size affects vigilance behaviour; 3) illegal hunting influences group size and vigilance behaviour; and 4) group size differs in different habitats, being larger in woodlands during the wet season and move around riverine habitats during the dry season. A negative relationship between the ratio of adult males to females with calves and group size was observed. The proportion of vigilant individuals was found to decrease in larger groups, and the proportion of vigilant individuals was higher in areas with high risk of illegal hunting as well as in groups with calves; and vigilance in males was low in small/singleton groups. However, the results do not support the hypotheses that illegal hunting, seasonality and habitat preference affect giraffes' group sizes in the ecosystem studied.