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This paper presents validation of the treatment of secondary stresses withinRevision 4 of the R6 assessment procedure using finite element analyses of crackedcylinders under thermal loading. Cylinder with a fully-circumferential internal crack and apart-circumferential external surface crack are considered as model examples. The crackdriving force values obtained from cracked-body finite element analyses are compared tovalues obtained using simplified uncracked-body routes within R6. The secondary stress isapplied using a temperature gradient. Various temperature loadings and crack sizes havebeen examined. Also, work performed by the Central Research Institute of Electric PowerIndustry (CRIEPI) in Japan, which has examined the same geometries, has been comparedto the present finite element results in some instances. It is concluded that for the cylinderwith a fully-circumferential internal crack the R6 routes are conservative when the thermalload is applied as an axially linear or radially non-linear temperature distribution. The R6routes are generally conservative when the thermal load is applied as a radially lineartemperature distribution, and is only very slightly unconservative in some of the cases.Good agreement is obtained between the present work and the CRIEPI data.