Model based process improvement has been successfully applied for software organizations. After observing this success, customizing it to specific domains/sectors becomes one of the most critical challenges. On the other side, there are some quality problems in government domain as: low efficiency and process performance, employee and citizen dissatisfaction, high cost and defect rates. Assuming that successful processes will be reflected in higher quality, the government processes have to be improved. However known capability/maturity models don't deal with particularities of government domain. Therefore, it's necessary to design and evaluate a domain-specific capability model. Towards this goal, we develop Government Process Capability Determination Model based on ISO/IEC 15504. Moreover, we propose a method which is a disciplined guidance for governmental organizations to perform process capability assessment systematically. The method is described in detail in the study.