Objective to explore a practical magnatic resonance imaging (MRI) method to predict the collapse of osteonecrosis of the femoral head (ONFH). Method 37 patients with ONFH in ARCO stage I or II were adopted. The mean follow-up period was 25 months. The angles of necrotic area in the Mid-coronal and mid-sagittal image were obtained to calculate the index of necrotic extent by the formula. The difference of the index between collapsing and non-collapsing group was compared. According to the index, patients were divided into high-risk, middle-risk and low-risk group in order to predict the collapse risk of ONFH. Result the index of necrotic extent in collapsing and non-collapsing group was 56.13±17.65 and 18.34±10.87 respectively with significant difference. No patient collapsed in the low-risk group with index less than 30 but all collapsed in the high-risk group with index more than 40, 45% patients in the middle-risk group with index between 30 and 40 collapsed. Conclusion The index of necrotic extent by MRI is the useful method to predict the collapse risk of ONFH.