Isobutanol is a flammable compound that can be used as a biofuel due to its high energy density and suitable physical and chemical properties. In this study， we examined the capacity of engineered strains of PCC 6803 containing the α-ketoisovalerate decarboxylase from and different heterologous and endogenous alcohol dehydrogenases (ADH) for isobutanol production. A strain expressing an introduced without any additional copy of ADH produced 3 mg L OD isobutanol in 6 days. After the cultures were supplemented with external addition of isobutyraldehyde， the substrate for ADH， 60.8 mg L isobutanol was produced after 24 h when OD was 0.8. The in vivo activities of four different ADHs， two heterologous and two putative endogenous in ， were examined and the endogenous ADH encoded by showed the highest efficiency for isobutanol production. Furthermore， the strain overexpressing the isobutanol pathway on a self-replicating vector with the strong P promoter showed significantly higher gene expression and isobutanol production compared to the corresponding strains expressing the same operon introduced on the genome. Hence， this study demonstrates that endogenous AHDs have a high capacity for isobutanol production， and identifies encoded α-ketoisovalerate decarboxylase as one of the likely bottlenecks for further isobutanol production.