Résumé : One of the cause of corrosion is the attachment of bacteria or commonly called as bio-corrosion or Microbial Influenced Corrosion (MIC). This aim of the research was to determine effect of heat treatment process on the bio-corrosion rate of API 5L steel. The treatments were namely, without heat treatment as a control, and with the heat treatment (austempering process). The austenizing process was conducted before the austempering process. All specimens without and with the heat treatment were be used on bio-corrosion test. The bio-corrosion testing was conducted with immersion corrosion test method with artificial seawater salinity of 35‰. Three of species bacteria were be used, Escherichia coli, Pseudomonas fluorescens, and Thiobacillus ferrooxidans. The result showed the corrosion rate on API 5L steel without bacteria was 2.7558 mpy, but it reached 3.4273, 3.6062 and 3.7699 mpy after addition with E. coli, P. fluorescens, and T. ferrooxidans, respectively. It was indicating that bacteria can accelerate the corrosion rate. The highest bio-corrosion rate due to T. ferrooxidans without heat treatment process was 3.7699 mpy. Meanwhile, the bio-corrosion rate due to T. ferrooxidans with austempering process was 3.5046 mpy. It was indicating that heat treatment can decrease the bio-corrosion rate.