The principal objective of this research was to study the morphological changes of Jack pine (Pinus banksiana) earlywood (EW) and latewood (LW) in thermomechanical pulping (TMP). The results indicate that EW fibers tend to separate at the P/S1 interface, whereas LW fibers commonly fail in the regions between the primary wall and the transition layer or outer layer of the secondary wall (P/S1) and between the outer and central layers of the secondary wall (S1/S2). LW fibers exhibit mostly intrawall failure and lower curl and kink indices, whereas EW fibers tend to fail in transwall mode (splitting) and suffer fiber cuttings. In addition, the thin-walled EW fibers show higher collapsibility and conformability than their thick-walled LW counterparts. Moreover, EW fines have higher surface lignin coverage, whereas LW fines have a higher specific volume (SV).