Abstract: For an unstable-first-order-plus-dead-time plant, the PI tuning formulae based on the conventional stability margin specifications can obtain the feasible stability margin region, however, only the increasing gain margin has been considered. The decreasing gain margin is obtained by Pade approximation of the delay in the framework of the previous tuning rules. The numerical polynomial solving approach is employed to seek the feasible stability margin region. At the same time, a novel tuning rule based on the decreasing gain margin is proposed. The results demonstrate that the feasible region is a small portion of the original one. Finally, some numerical examples are presented to validate the results.