In this study, heart rate variability measurements and analyses were carried out with help of the ECG recordings to show how autonomic nervous system activity changes. ECG signal recording was collected from six volunteer participants under following conditions; the situation of relaxation, Stroop color/word test, mental test and auditory stimulus in a laboratory environment so as to evaluate the parameters related to stress. Totally seven minutes ECG recording was taken and analyzed in time and frequency domain. We investigated that how autonomic nervous system activity changed in the position of stress. According to frequency domain analysis, there has seen significant increase at low frequency band (LF band) which reflects sympathetic activity particularly during Stroop color/word and mental tests.