Al2O3/h-BN machinable composites were cost-effectively fabricated by pressureless sintering method. The machinability, deformation, and cracks behavior of the composites were investigated by drilling, Hertzian indentation, and Vickers indentation test, respectively. Through the observation of the microstructures in different scales by SEM, we analyzed the role of the weak boundary phases (WBP), including h-BN and pores, on the machining mechanism of the composites. The results showed that almost all of the WBP dispersed at the Al2O3 grain boundaries, which strongly elevate the machinability and deformability of the composites. During the drilling or Hertzian indentation test, a large number of microcracks formed firstly along the WBP; then these microcracks connected with each other causing a removal or macro-deformation of the composites. The Vickers indentation test result indicated that the weak interfaces and grain boundaries of WBP lead to a low microcrack toughness, whereas in macro-scale the crack toughness was improved because of the crack bridging and deflection of WBP.