In a 2N‐pole electromagnetic system, defining the electron optical Hamiltonian function, we have derived both general algebraic expressions (arbitraryN) and special numerical formulas (N=3,4,5,6,7) for different aberrations from lower to higher order (i.e., the order ofN−1,N+1, 2N−3, 2N−1, 3N−5). The so‐called canonical aberration theory in electromagnetic multipoles has thus been developed, which allows us to deduce angular dependencies of different aberrations and to examine the possibility for spherical correction of a round lens by using multipoles.