This paper introduces a procedure to assess the credibility of models and simulations (M&S) as a group activity based on NASA's new standard for M&S NASA-STD-7009. The Delphi method, which is characterized by iterative surveys with controlled feedback, was selected to implement the assessment. The proposed procedure is expected to address the issues in the M&S assessment related to a high level of required expertise and group decision making. An actual credibility assessment study using the proposed procedure on an M&S platform referred to as SpaceNet has been carried out by ten panel members through a two-round Delphi. The study concluded that the overall credibility of SpaceNet version 1.3 was between the development level and production level. The variances of the assessments in the second-round survey were significantly reduced compared with the first-round results, which indicates the effectiveness of the proposed procedure.