The strain effect on the critical current density, Jc, of NbN tapes has been investigated in 4.2K and 9T. Superconducting films of NbN have been prepared on metal tapes of Hastelloy B and 304 Stainless steel by reactive sputtering in an argon-nitrogen atmosphere. The strain dependences of Jc are influenced by the substrate material and NbN film thickness. The NbN tapes with Hastelloy B substrate maintains constant values of Jc up to strain values of above 1%. However, those with 304 Stainless steel substrate show a gradual decrease in Jc from 0.3 to 0.9% and exhibit a marked decrease in the range of above -0.9%. The value of the irreversible strain limit εirrev. where each NbN tape is permanently damaged, is in good agreement with the ratio between the proportional limit and Young's modulus in an investigated stress-strain curve. The maximum value of εirrev. reaches -1.4% and has been obtained in the tape consisting of the Hastelloy B substrate and a 3, 000Å NbN film. In tapes thickly coated with NbN, which have been measured up to above their εirrev. values, some cracks are found and films of NbN are frequently peeling off their substrates.