@ysmina-- How severe is the rash? Zinc oxide ointment for diaper rash is suitable for mild rashes. If the rash is severe, you will probably need a stronger treatment. Even hydrocortisone cream might be necessary sometimes but ask your doctor first.
If you can find one, purchase a zinc oxide ointment with lanolin. Lanolin is a natural moisturizer. This combination seems to work better than just zinc oxide ointment.