Patterned magnetic dots were fabricated by an Artificially Assisted Self-Assembling (AASA) method, and the dot size dispersion was estimated. In AASA method, self-assembled polymer dots are formed in a groove guide and are used for the etching mask. It is found that the dispersion of the size of the magnetic dots (average diameter: 23nm, average pitch: 45nm) can be reduced by optimizing the guide shape. Decreasing the molecular weight of the polymer is also found to be effective to reduce the dot size dispersion. Dispersion of 10.6 % was achieved using a hexagonal guide. This feature is advantageous to the patterned magnetic recording media compare to the electron beam lithography, where the dispersion is increased with the patterning size.