This research work is aimed at investigating the effect of biocide injection in the treatment of internal corrosion of Oil pipelines. in this research. carefully formulated laboratory experiments are performed on a sample 017 sump) tank discharge from Mobil Producing Nigeria (MPN) offshore facilities using formaldehyde (methanol) as the biocide. This is to assess its effect on bacterial proliferation as well as hydrogen sulphide (H2S) Formation, under both aerobic and anaerobic conditions. The result from the turbidity observed if) Untreated cultures showed that bacterial growth had occurred in them and such growth must have been hindered by the formaldehyde in the treated aerobic and anaerobic cultures. A blackening of lead acetate paper observed with emanating rotten egg smell gas is an indication For the formation of hydrogen sulphide gas. Biocide (formaldehyde) has proved to be efficient at combating bacterial growth and hydrogen sulphide production by the bacteria. These results show that biocide can be used to prevent internal corrosion of oil pipeline since it inhibits hydrogen sulphide formation from metabolism of the bacteria.