Drosophila male germline stem cells (GSCs) reside at the tip of the testis and surround a cluster of niche cells, called the hub. It has been believed that the Decapentaplegic (Dpp) ligand, secreted from the hub, only activates stem cells in close proximity, but not differentiating cells spaced one-cell layer away. However the range of Dpp diffusion is unknown. Here, using genetically encoded nanobodies, called Morphotrap, we physically block Dpp diffusion outside of the niche without interfering with niche-stem cell signaling. Surprisingly, we found that Dpp has an opposite effect on GSCs and on their immediate progenies, such that it promotes self-renewal of GSCs, while ensuring the differentiation of their daughter cells. When the signal from the diffusing fraction of Dpp was specifically blocked, differentiating daughter cells frequently dedifferentiated, suggesting that Dpp ensures asymmetric outcome of stem cells both inside and outside of the niche. We further show that these distinct signaling outcomes are achieved by the same canonical BMP pathway, through the typeI receptor, Thickveins (Tkv), and the downstream effector, Mothers against Dpp (Mad). Given the broad requirement of BMP pathway in many stem cell systems, we propose that such a self-contained behavior of a stem cell ligand may be a common mechanism to ensure special restriction of stem-cell niche.