Background:The determination of the width of the implant is the first key step to select shape and volume of the implant in breast augmentation.The aim of this study was to introduce a new method to determine the width of the implant (W) and explain the reasons to do so in details.Methods:From January 2006 to June 2014,the authors have found and applied CD-4 theory to determine the width of breast implant (W) in dual plane Ⅰ or Ⅱ breast augmentation cases through transaxillary or periareolar incision for 560 patients.CD is defined as the curved distance on skin from the midline of the stal bone to the anterior axillary line (AAL) on the lateral chest wall through the horizontal level on inferior mammary fold.W =CD-4 (or 3.5) cm.Results:The 560 patients used both round and anatomic implants with W from 10.5 cm to 12.5 cm.Their CDs are from 14.5 cm to 17 cm.About 78％ of the patients have got followed up from 1 month to 5 years postoperatively.Except for four patients who got unilateral capsular contractions,all the other patients are satisfied with their nature new breast shapes and volumes.Their new intermammary cleavages without bras are between 1 cm and 2.5 cm,and lateral borders of the breast are on the area of the AAL.Conclusions:W (width of the implant) =CD-4 (cm) when doing dual plan Ⅰ or Ⅱ breast augmentation.For the very thin patient,4 should be 3.5.