A steel-concrete composite slim beam, with its steel section encased in a concrete slab, provides a composite steel-concrete floor construction with minimum depth and high fire resistance. It has become popular in multi-storey steel buildings throughout Europe in recent years and also been used in some projects in China. For the structural engineer, it is critical to understand the behavior of composite slim beam in the sagging moment region. In this paper, the bending capacity and flexural stiffness of the composite slim beams in sagging moment region are investigated theoretically. Meanwhile, the stiffness and bending capacity are given by Finite Element Analysis (FEA). The experiment of a simply supported beam has been carried out in order to validate the theoretical results.