Ultra-short-period planets ( USPs ) provide important clues to planetary formation and migration .
Recently , it is found that the mutual inclinations of the planetary systems are larger if the inner orbits are closer ( \lesssim 5 R _ { * } ) and if the planetary period ratios are larger ( P _ { 2 } / P _ { 1 } \gtrsim 5 ) ( ) .
This suggests that the USPs experienced both inclination excitation and orbital shrinkage .
Here we investigate the increase in the mutual inclination due to stellar oblateness .
We find that the stellar oblateness ( within \sim 1 Gyr ) is sufficient to enhance the mutual inclination to explain the observed signatures .
This suggests that the USPs can migrate closer to the host star in a near coplanar configuration with their planetary companions ( e.g. , disk migration ) , before mutual inclination gets excited due to stellar oblateness .