How cancer cells respond to different mechanical environments remains elusive. Here, we investigated the tension in single focal adhesions of MDA-MB-231 (metastatic breast cancer cells) and MCF-10A (normal human breast cells) cells on substrates of varying stiffness using single-cell measurements. Tension measurements in single focal adhesions using an improved FRET-based tension sensor showed that the tension in focal adhesions of MDA-MB-231 cells increased on stiffer substrates while the tension in MCF-10A cells exhibited no apparent change against the substrate stiffness. Viscoelasticity measurements using magnetic tweezers showed that the power-law exponent of MDA-MB-231 cells decreased on stiffer substrates whereas MCF-10A cells had similar exponents throughout the whole stiffness, indicating that MDA-MB-231 cells change their viscoelasticity on stiffer substrates. Such changes in tension in focal adhesions and viscoelasticity against the substrate stiffness represent an adaptability of cancer cells in mechanical environments, which can facilitate the metastasis of cancer cells to different tissues.