In Brassica, self-incompatibility (SI) can be overcome by $CO_2$ application, an effective method for obtaining numerous inbred lines for $F_1$ commercial seed. We previously reported two different S-alleles of Brassica campestris, $S^{733};and;S^{734}$, with extremely different degrees of susceptibility to this gas. In the current study, we raised a cross-population between those two genetic lines, and analyzed their reaction level of self-incompatibility to $CO_2(RLSICO_2)$. Here, all 40 of our progeny from the $F_1$ cross-population were susceptible, maintaining high values of $RLSICO_2$. This suggests that the susceptible line, $S^{734}$, is dominant to the insusceptible line, $S^{733}$. We also generated an $F_2$ selfing-population of each crossed progeny, $S^{733}female;S^{734}male;and;S^{733}maleS^{734}female$, to assess the $RLSICO_2$ of each individual. PCR-RFLP analysis was performed to determine the S-genotype of the $F_2$ population. The $S^{734}$ allele segregated in a theoretical ratio of the dominant trait, and the $RLSICO_2$ was consistent with the dominance relationship. Therefore, we have now demonstrated that high $RLSICO_2$ in B. campestris is controlled by a dominant gene.