This study classified oyster shells that produced as a industrial waste into 3 distribution by washing, drying and processing them. Mortar specimens with a constant ratio by using this to substitute fine aggregates were made, and the specimens were heated under the heating conditions of 300℃, 600℃ and 900℃ based on the 28-day age. On the age of 28 days, the plain flexural strength was found to be 9.2MPa, and in O 0.15, it was shown to be 4.4∼ 7.9MPa depending on the substitution rate. It was found to be 4.4∼7.7MPa in O 1.2∼2.5 depending on the substitution rate, and last but not least, it was shown to be 6.1∼8.8MPa in case of O 2.5∼5.0 depending on the substitution rate. In case of the compressive strength of the 28-day age, it showed the difference of 23.6∼43.2MPa in O 0.15 depending on the substitution rate, and 20.4∼45.1MPa in O 1.2∼2.5 depending on the substitution rate, and last but not least, 17.1∼40.4MPa in case of O 2.5∼5.0. As a result of measuring the residual strength through heating, in case of substituting fine aggregates less than O 0.15 by 100%, it showed the lowest strength reduction ratio, and it is expected that the heat-resisting property could be achieved through processing and proper mixing of oyster shells through these results.