Lithium metal is considered the ultimate anode material for future rechargeable batteries(1,2), but the development of Li metal-based rechargeable batteries has achieved only limited success due to uncontrollable Li dendrite growth(3-7). In a broad class of all-solid-state Li batteries, one approach to suppress Li dendrite growth has been the use of mechanically stiff solid electrolytes(8,9). However, Li dendrites still grow through them(10,11). Resolving this issue requires a fundamental understanding of the growth and associated electro-chemo-mechanical behaviour of Li dendrites. Here, we report in situ growth observation and stress measurement of individual Li whiskers, the primary Li dendrite morphologies(12). We combine an atomic force microscope with an environmental transmission electron microscope in a novel experimental set-up. At room temperature, a submicrometre whisker grows under an applied voltage (overpotential) against the atomic force microscope tip, generating a growth stress up to 130 MPa; this value is substantially higher than the stresses previously reported for bulk(13) and micrometre-sized Li-14. The measured yield strength of Li whiskers under pure mechanical loading reaches as high as 244 MPa. Our results provide quantitative benchmarks for the design of Li dendrite growth suppression strategies in all-solid-state batteries. Lithium whisker growth and mechanical properties can be studied in situ using a combination of two microscopies.