The objective of this paper is to assess the effect of Bt (Bacillus thuringiensis) varieties on cotton yield and the amount of insecticides used in cotton field in Burkina- Faso upon the adoption of Bt cotton after 2005. In the paper, we use a simple version of a Muth model to assess the welfare effect of the adoption of Bt cotton in Burkina- Faso. Using the survey data from SOFITEX, SOCOMA and Faso Coton, we consider a single output and five inputs under the conditions of competitive markets. The results show a rise in yield and a reduction in the amount of insecticides used upon the adoption of Bt cotton in Burkina-Faso. In terms of welfare effect , an increase in the consumer surplus, in the producer surpluses of Seed company, fertilizer and herbicide supplier, workers and “ land owners”; but a decrease in the producer surplus of the insecticides suppliers are found.