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Nowadays, the implementation of sophisticated in situ electron microscopy tests is providing new insights in several areas. In this work, an in situ high-temperature strain test into a scanning electron microscope was developed. This setup was used to study the grain boundary sliding mechanism and its effect on the ductility dip cracking. This methodology was applied to study the mechanical behaviour of Ni-base filler metal alloys ERNiCrFe-7 and ERNiCr-3, which were evaluated between 700°C and 1000°C. The ductility dip cracking susceptibility (threshold strain; εmin) for both alloys was quantified. The εmin of ERNiCrFe-7 and ERNiCr-3 alloys were 7.5% and 16.5%, respectively, confirming a better resistance of ERNiCr-3 to ductility dip cracking. Furthermore, two separate components of grain boundary sliding, pure sliding (Sp) and deformation sliding (Sd), were identified and quantified. A direct and quantitative link between grain boundary tortuosity, grain boundary sliding and ductility dip cracking resistance has been established for the ERNiCrFe-7 and ERNiCr-3 alloys.