The fracture energy, toughness and failure modes of air plasma oxidized polydimethylsiloxane (PDMS) bonding are evaluated in this paper. Each PDMS-PDMS bonded specimen was subjected to T-peel testing at a constant-displacement rate. The load-extension curve from each test was analyzed by the principle of energy balance in linearly elastic fracture mechanics, to calculate the fracture energy (critical strain energy), which is the maximal strain energy a bonded specimen can withstand without losing its assembly integrity, and toughness (surface energy), which is the bonding energy at the interface. A distribution of calculated values against the air plasma treatment parameters shows the predominant range of 0.1 to 0.4 N/mm for fracture energy and 0.1 to 0.2N/mm for toughness. Together with an analysis of three failure modes (cohesive, adhesive, and mixed), the results suggest 0.1N/mm as the threshold of the fracture energy for weak bonding, below which a specimen will be likely to fail through debonding. A set of treatment parameters are recommended for using air plasma to achieve strong bonding.