The corrosion nature of mild steel in Rajshahi University tap water along with distilled water and acid water have been investigated by immersion test and surface analysis. The comparative corrosion rates have been calculated based on the mass loss due to immersion in the medium. In some cases, corrosion has been found to be higher in distilled water than in tap water. It is believed that tap water contains some metal ions that play a special role in preventing corrosion in aqueous medium. Same surface morphologies were observed of specimen immersed in tap water and distilled water. However, numbers of pits and grain boundaries were observed on the specimen immersed in acid water indicated that severe corrosions were occurred. Rajshahi University tap water mostly turbid and contains different ions specially, iron and chloride ions. Due to having some aggressive ions initially corrosion rate is increased then decreased consistently with time for the presence of metal cations. If the water is filtered then some unwanted matters and aggressive ions are removed and corrosion rate also declined a bit. However, this dirty water become useless in the laboratory as well as in the washroom. On the other hand, corrosion rate in distilled water was found to be higher than tap water. Because distilled water is free from aggressive ions as well as metal cations.