This study is concerned with transitivity analysis of selected Nigerian senior high school biology textbooks. The aims of this research were to describe and to find out the dominant process types of transitivity occurrence, reason behind the choice of dominant process, and its effect in the biology practical instructions. This study was descriptive qualitative. The data were obtained from the selected two Nigerian biology textbooks entitle Essential Biology for Senior Secondary Schools, and Modern Biology for Senior Secondary Schools by M. C Michael, and Ramalingam respectively. The result of the research was four types of process that occurred in the textbooks. There was material process (90.83%), mental process (4.59%), relational process (3.67%), and existential process (0.92%). The dominant type of transitivity is material process. It means that biology practical instructions textbooks contained action and physically. Furthermore, the benefit that can be gained from analysing the transitivity in the text could stimulate the reading skills to comprehend the idea and information in a text that could be applied in spoken and written texts. Based on the finding of the transitivity analysis, it was recommended that, serving teachers be trained through workshops, seminars, conference s and in-service programmes.