Design delays can adversely affect the total completion time of a construction project. Factors affecting the delays of design duration are complicated and interrelated. This study proposes a methodology to support identify key driving factors affecting design delays and sieve out the initiating delay factors for improvement. The core of the methodology is to integrate a satisfied importance analysis and a decision making trial and evaluation laboratory technique. A real-world design project in Taiwan is applied to examine the benefits of the methodology. In this case study, four first-level delay factors and 17 second-level delay subfactors are defined. The organizations decision makings and budget constraints is identified as the key driving factor causing design delays in this case project. Top management of the case project appreciates the application results.