A quantitative tool is necessary in evaluating and deciding the flexibility in the enterprise knowledge management system (EKMS).  The components of the flexibility, the subjective and objective factors causing the environment uncertainty and the meaning of the fitting of the flexibility to the environment uncertainty were discussed.  The evaluation index system and the measurement method for the flexibility and the environment uncertainty are proposed.  From spacial geometry, the method to compute the fitting and its judgment criteria were designed.   An example of the computation is also given.