Using the archival ROSAT PSPC observations, AB Dor is found to be variable in X-rays. The periodic variations are consistent with previously reported rotational period of 0(d).514. The average spectrum of AB Dor is best represented with two-temperature Raymond-Smith model with kT values of 0.19 +/- 0.07 and 1.17 +/- 0.02 keV. The quiescent luminosity of the system is found to be 4.36 +/- 0.6 x 10(30) ergs s(-1). A flare with a rise time of similar to 350 seconds is detected during which X-ray luminosity rises from 5.8 +/- 1.6 x 10(30) to 15.8 +/- 4.9 x 10(30) ergs s(-1). We conclude that AB Dor is very similar to the active components of RS CVn binaries and other active classes. In view of the wide separation from the binary companion Rst 137B, this activity must be intrinsic to the active star.