Electrochemically charging induced phase transition is a common and thermodynamically-driven phenomenon for variety of cathode materials, which couples with chemical and mechanical effects leading to performance degradation. Phase transition is particularly complex for layered sodium transition metal oxides and its related detrimental effects remain elusive. Herein, we take P2-type Na2/3Ni1/3Mn2/3O2 (P2-NNM) as an example to scrutiny the detrimental consequences upon high voltage cycling. We find that repeated P2-O2 phase transition breaks down cathode primary grains by generating high density of intragranular cracks, which is qualitatively proved to be the main cause of performance decay. Intriguingly, the nucleation and growth of intragranular crack is through loss of atoms rather than cleavage, resembling the stress corrosion cracking mechanism which preferentially nucleates at P2/O2 phase boundary. Moreover, we find the P2-structured cathode is not sensitive to surface degradation, which explains the superior performance of P2-NNM cathode when cycling at low voltage.