Mechanical and thermal properties of stereolithography 3D-printed thermoset polymers have been investigated with an emphasis on understanding how the layer-by-layer printed morphology affects their physical properties. Due to the finite UV penetration depth into the photocurable resins in the stereolithography, the 3D-printed polymers resulted in periodic surface undulation. The length scale of the surface undulation periodicity is determined by the 3D printing processing parameter of slice thickness (t) and has a strong effects on the mechanical and thermal properties of the 3D-printed thermoset polymers. Upon decreasing the t from 200 m to 50 m, the 3D printing produces more frequent UV curing of crosslinked layers along the printing direction to result in the increase of Young's modulus and the more pronounced high glass transition peak in dynamic mechanical analysis. However, when the t is further decreased to 25 m, its Young's modulus is lower than the 50 m printed samples. This decrease is attributed to the weaker inter-layer crosslinking, when there are not enough less-cured monomers to promote the crosslinking between densely crosslinked layers during the successive layer-by-layer photopolymers in stereolithography.