Determination of radioactivity of natural samples of water, air and soil has always been considered in research centers in different parts of the world. In this Work, the amounts of natural radioactive materials 238 U, 232 Th, 40 K as well as 137 Cs, which is produced artificially in human nuclear activities, have been measured from the hot springs of Mahallat ( in central part of Iran) and also in a number of available mineral water samples. The measurements have been performed by gamma-ray spectroscopy using high-purity germanium detector (HPGe), multi-channel analyzer and Gamma vision software. Ten water samples were collected and each sample was measured for 162000 seconds (or 45 hours). The values obtained were in the range of 2.39 to 4.49 Bq/lit for 238 U, 1.48 to 3.57 Bq/lit for 232 Th, 7.51 to 41.99 Bq/lit for 40 K and 0.014 to 0.15 Bq/lit for 137 Cs. These values are comparable with those found in other parts of the world and they are all well below natural limits. The Radium Equivalent quantity was also calculated for the samples and was found to be in the range 5.91 to 11.23 Bq/kg which is well under maximum allowed value of 370 Bq/kg. External and Internal Hazard Indices were also determined for the samples and were found to be in the range of 0.0159 to 0.033 Bq/kg for External Index and 0.0276 to 0.0419 Bq/kg for Internal Index. Which are well below the maximum allowed value of unity for each index.