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The Inter Granular Stress Corrosion Cracking (IGSCC) is a dominant damage mechanism of the austenitic stainless steel. The primary circuit piping of RBMK type reactors is produced from austenitic stainless steel 08×18H10T. Defects in welded joints of pipes with nominal diameter of 300 mm were detected during In-service inspections [1]. Metallographic investigations defined that crack growth mechanism is IGSCC. The appearance of defects increases the probability of RCS piping failures of these pipes. A leak or break in RCS piping is not acceptable from safety and political (society risk) points of view. According this the evaluation of these cracks is very important for safe operation of this type reactor. The procedures for IGSCC crack evaluation consist of two parts. The first part is determination of the acceptable crack size for the component with crack, and the second part is the crack growth calculation. The acceptable flaw size provides information about the largest flaw size which component can tolerate without failure with accepted safety factors. The crack growth calculation determines how long does it take for the existing crack to reach the maximal acceptable size. The results of these calculations (acceptable crack size and crack growth) determine the further inspection schedule of the components with crack. The objective of this paper is the evaluation of the IGSCC defects detected during In-Service inspection in the primary circuit piping which outside diameter of piping is 325 mm, the wall thickness – 16 mm. Detected cracks were evaluated using method R6 [2]. The IGSCC crack growing analysis was performed using methodology presented in document [3]. The prognosis results were compared with crack data detected during In-service inspection. According analysis results were determined that the IGSCC defects detected during In-service inspection can be left without repairing for 1.5 years operation.