Granger causality analysis is applied to very-low-frequency (VLF) waves and relativistic electrons interaction in an outer Van-Allen radiation belt. The VLF wave power and electrons flux data are one hour observations of radiation belt storm probe A (RBSP-A) during 14-3-2015. The considered VLF band comprises of 24 waves and electrons flux data at 8 pitch angles is considered. It is found that every wave is not Granger causing flux gradients for all pitch angles; rather some VLF waves are more efficient than others. Specifically, flux gradients at 69 degrees pitch angles are Granger caused by 14 kHz VLF wave. Contrarily, for 90 degrees pitch angles flux gradients are Granger caused by 6 waves. These results may be helpful for understanding the dynamics of outer radiation belt.