The time variability of diurnal tides was investigated by analyzing gravity observations from global super-conducting gravimeter (SG) stations with running time intervals. Through least-square and Bayesianapproaches, FCN resonance parameters were estimated for each data section after obtaining the tidalparameters of mainly diurnal tidal waves. The correlation of the time variation in diurnal tidal waves andFCN period was discussed. For comparison, a similar method was used to analyze VLBI observations tostudy the time variability of nutation terms and FCN period. The variation trend of the FCN period totallydepends on the 1 wave in tidal gravity and on the retrograde annual term in nutation. We observed asimilar variation trend in the FCN periods obtained from different SG stations worldwide and VLBI obser-vations. The relation between diurnal tides and LOD variations is discussed and the possible mechanismsof the decadal variation in FCN periods were discussed.