Noticing the ever increasing dependency of life to the electrical energy, continuance of high quality service non-failure of power is very important for consumers. For achieving this goal, each main part of power system should have proper reliability. Some Papers have introduced the modeling methods for reliability evaluation of Transferring and Distribution parts at the power systems, But this paper for the first time presents the analysis of reliability indices for all parts of generation unit (Thermal Power Plants) by using new method of modeling. The Unit Indices in different states of power limitations and power not-supplied can be calculated. It can also be calculated the total average of annual not-supplied energy on the basis of occurred different errors. The effect of time changing of equipment repairs, that is equal to adding cost and/or personnel number, on the Unit Reliability improvement, is surveyed. Personnel errors and Repair costs are decreased, generation income is increased, by training of personnel and spending less costs. Sensitive equipments can be identified therefore investment is propelled toward removing probable errors of these equipments. The effect of each error on generation system indices are studied on the basis of the kind of error.