Microcracking susceptibility in the dissimilar multipass weld metal of alloy 690 and low-alloy steel A533B was evaluated, and the effect of dilution on hot cracking (ductility-dip and liquation cracking) behavior was investigated. In order to simulate the dissimilar multipass weld metal of alloy 690 to A533B steel, the A533B plate was welded under the various dilution ratios using the alloy 690 filler metal with different contents of P and S. The several weld metals, which had different alloy compositions at the fixed (P+S) content, were manufactured, and then ductility-dip and liquation cracking susceptibilities of the reheated weld metals were evaluated by the spot-Varestraint test. The ductility-dip cracking susceptibility heightened as the dilution ratio was increased even when the amounts of P and S were fixed. The increased dilution ratio (contamination of Fe into the weld metal) should reduce the tortuous character of the grain boundary due to inhibiting the constitutional supercooling (the instability of the solidification boundary), as well as enhance the grain boundary embrittlement due to promoting the grain boundary segregation of P and S. Furthermore, the liquation cracking susceptibility slightly heightened with an increase in the dilution ratio at the fixed (P+S) content. The increased liquation cracking susceptibility would be attributed to the enhancement of solidification segregation of P and S with increasing the dilution ratio.