•T4 treatment improves the corrosion fatigue strength.•Localized corrosion and cracked TBs compete for the fatigue crack initiation.•T4 treatment alleviates the corrosion attack and weakens the HE.•Cracking at TBs reduces the corrosion fatigue lifetimes of T4 samples. In this work, the corrosion fatigue behaviors of an as-forged Mg-6.7%Zn-1.3%Y-0.6%Zr alloy before and after solid solution treatment (T4) in 3.5 wt% NaCl solution have been investigated and compared. S-N curves showed that the fatigue strength of as-forged samples corresponding to 5 × 106 cycles was 30 MPa, whereas the fatigue strength of T4 samples was 50 MPa. Fracture observations showed that for the as-forged samples, fatigue cracks mainly initiated at localized corrosion sites on sample surfaces. However, for the T4 samples, the crack initiation was respectively related to the localized corrosion and TB cracking at low and high stress amplitudes.