Weaver ants Oecophylla smaragdina F. were sampled and studied for 2 years on Chiang Mai University campus. Nests of weaver ants were found mostly from April to June on many different shrub and tree species. Highest peak of population was in February showing the swarming of females and males. Reproductive castes were found in the nests from January to July. After the nests were settled again, worker castes increased to take care of the larvae. Amount of large worker was higher than small workers. Because of the very small size of the eggs and short egg stage, they were rarely found female larvae castes were bigger than other castes. Pupae and adults of the 4 castes were different.