Ulnar collateral ligament (UCL) injuries are becoming increasingly common. The palmaris longus (PL) and gracilis (GR) tendons are the most common grafts used in UCL reconstructions. While clinical studies have demonstrated relatively similar outcomes for either graft, there is little quantitative data describing these grafts from a material perspective, specifically the mechanical and microstructural properties of these tissues and how they respond under dynamic loading. The purpose of this descriptive laboratory study was to quantify and compare the mechanical and microstructural properties of PL and GR tendons. A total of 13 PL and 11 GR cadaveric human tendons were obtained. Each specimen was divided into three subregions and subjected to preconditioning, ramp-and-hold stress-relaxation and ramp-to-failure testing. Mechanical parameters were computed for each sample, and a polarized light imaging technique was used to simultaneously evaluate dynamic microstructural properties during testing. The PL had larger toe- and linear-region modulus values than the GR. Within the GR, the distal subregion had stronger collagen alignment than the proximal subregion at the zero, transition and linear portions of the stress-strain curve. The PL and GR, have similar mechanical properties and similar microstructural alignment under load. The PL graft has similar properties throughout its length whereas the GR properties exhibited slight differences in strength of alignment along its length. The PL and GR exhibit larger moduli values and more strongly/uniformly aligned collagenous microstructure when qualitatively compared to data previously published on the native UCL.