In a method of forming a shallow trench isolation (STI) region in a semiconductor device, a pad oxide layer and a pad nitride layer may be formed on a semiconductor substrate. The pad nitride layer and pad oxide layer may be patterned to form an isolation region with exposed portions on the pad nitride layer, pad oxide layer and semiconductor substrate. A radical oxide layer may be formed on the exposed portions, and a trench may be formed in the isolation region by etching the semiconductor substrate and radical oxide layer. The STI region may be formed by filling an insulating layer in the trench.