System thinking ability; enables to consider events, states or a problem not only as a whole but also to see them both in pieces and to observe the relation among the pieces. At this point it is thought that system thinking ability has an important place in terms of both education environments and for our daily life. Within this study it is aimed to determine the states of secondary school sixth grade students regarding system thinking ability. Integrated single state study was used among research methods. The study was carried out in Bursa in 2015 and 207 secondary school sixth grade students participated in the study. Within the study, questionnaire form was used, which consisted of open ended questions regarding system thinking ability concerning systems in our body unit. Within the stage of analysis of the questions rubric, which was developed by the researchers, was used. It is understood that secondary school students' system thinking abilities were at low level in systems in our body unit especially within the dimension of making generalizations. As it is known making conclusions takes place among high level of thinking abilities and it is thought that it is hardly understood by the students.