The catalytic [5 + 1] annulation/5-exo-cyclization reaction of amidines with diynes is reported herein. This protocol provides highly atom-economical access to fabricate two nitrogen-containing heterocycles in one step with high efficiency and selectivity. Significantly, this reaction represents the first example of using diyne as a one-carbon reaction partner in C-H functionalization. Kinetic isotope effects suggested that the catalytic cycle of this reaction is initiated by the cleavage of the ortho C-H bond in the N-phenyl ring of amidines, which is likely involved in the rate-limiting step. Calculations based on density functional theory (DFT) indicated that C-H activation and the formation of Rh(V) species via 5-exo-cyclization could be vital processes for this cascade transformation.