Abstract: I will expain the process of conifold transition from Calabi-Yau threefolds to connected sums of S^{3}X S^{3}. Following the work of Y. Kawamata, G. Tian and R. Friedman, the existence of this kind of conifold transition is reduced to the existence of pairwise disjoint isolated rational curves in these Calabi-Yau threefolds. As an example, I will deal with the complete intersection Calabi-Yau threefolds in products of projective spaces.