Heat Recovery Steam Generator (HRSG) is the main equipment of the Steam-Electric Power Plant (PLTGU) which serves to utilize the exhaust heat energy from the gas turbine to heat the water into steam, then steam is used to drive the steam turbine. HRSG UP Grati has been operating for 21 years (since 1997) and has been frequently done minor repair due to damage such as superheater tube leakage, preheater, casing & ducting repair including heat insulation. In addition to its age or life time, the operating pattern of PLTGU Grati as a load follower requires that GT and HRSG operate in partial load conditions and even require start-stop if the gas supply is deficient. Operation with partial load and start-stop conditions causes the load on HRSG to be highly volatile. As a result of these operating patterns, resulting in the occurrence of fatique stres and deformation in pressure parts and mechanical structure of HRSG and leaks on HRSG tubes. To overcome these problems, the HRSG manhole design is made easier and can shorten the maintenance process time, because if the maintenance or repair is done with a long time then cause damage to spread to other parts and make greater losses due to malfunction HRSG . The methodology used is designing HRSG manhole using Solidworks software. Next, the calculation of technique includes calculation of force, frame strength, material, then analyzed. The result of planning obtained for HRSG manhole design at PLTGU Grati is as follows: manhole size 800x800mm, manhole material using stainless steel 304, manhole isolator material using Rockwool. With the manhole designed, it is expected to speed up the process of repair and minimize the gas leak.