Short-time Fourier Transform (STFT) provides an advantage of revealing the frequency contents of the signal at each time point in the signal. This information can be used to provide control and perform several tasks in Brain Computer Interface system. This paper describes the STFT analysis of EEG signals obtained during relaxing and writing, The results of the STFT analysis showed that there are significant differences in the frequency components and patterns between the EEG signals obtained from relax and writing.