The conditions for formation of ion acoustic weak double layer in a dusty plasma are investigated. In case of much lower value of ion-electron temperature ratio, the weak double layer is observed when the fraction of the negative charge in the plasma on the dust grain is ${\geq}0.45$ , whereas for equal ion-electron temperature it is observed only when the fraction is ${\geq}0.6$ . The parameter regimes of transition from compressive to rarefactive double layer are also specified.