South Sipilou is an area containing lateritic nickel mineralization that developed on ultramafic rocks in the department of Sipilou-Biankouma in western Ivory Coast. The objective of this study was to characterize the petrography and hydrothermal alterations affecting these ultramafic bed-rocks and to understand their involvement in lateritic nickel mineralization. Then to compare them to the ultramafic rocks already characterized in the department of Sipilou-Biankouma. Thus, the macroscopic characterization of these rocks was carried out in the field. Then, microscopic observations on the petrography and the hydrothermal alterations were made on these rocks after the preparation of thin sections in the laboratory.The results indicated that the lithologies of the ultramafic bed-rocks of South Sipilou consist of strongly serpentinized dunites, more or less harzburgitic lherzolites and olivine orthopyroxenites. Their petrographic characteristics showed a similarity with the ultramafic bed-rocks of the nickel-bearing lateritic mineralization of North Biankouma, North Sipilou and differences with those of Samapleu, Yepleu, in the department of Sipilou-Biankouma. Also, the main hydrothermal alterations of the ultramafic rocks of South Sipilou are composed of silicification, carbonation and serpentinization. Only serpentinizationcontributed to the concentration of nickel in the bed-rock and within the lateritic profile.