<p>The conformal cooling channels (CCCs) can improve the cooling efficiency of die casting molds, but improper designing might induce larger residual stress, especially for molds with diameter-varied CCCs. The effect of connection mode on the stress between two diameter-varied CCCs was studied by finite element analysis. The value and position of the maximum stress differ between the surface and the internal CCCs. For a fixed radius difference (<i>a</i>) of two diameter-varied CCCs, the increase of the length (<i>b</i>) of the transition region not only leads to the gradual decrease of the maximum stress, but also shifts the stress concentration position from the transition region between two diameter-varied CCCs to the parallel region with a smaller CCC diameter. Under the condition of a permanent wall thickness, the value of <i>b</i> should be at least twice of <i>a</i> value so as to avoid serious structural stress. The results of mechanical performance test for the 18Ni300 maraging steel, fabricated using selective laser melting method, verified the validity of simulation. Samples with the absolute value of <i>a</i>/<i>b</i> above 1/2 present brittle fracture behavior, otherwise possessing good tensile ductility.</p>