Let G be a reductive connected group over an algebraically closed field of characteristic p . Of particular importance in the study of G is the set u(G) of unipotent conjugacy classes. It is known that the cardinal of u(G) depends on p. We propose to define another set str(G) which is closely related to u(G) but is independent of p. But while u(G) is a subset of the set of conjugacy classes, str(G) is a quotient of that set. Its elements can be viewed as parts of a partition of G. We also show that the set of conjugacy classes in the Weyl group is partitioned according to str(G).