This study is to investigate the phase changes and cold workability after isothermal transformation at $780^{circ}C$ by using the high temperature gas nitrided (HTGN) STS 430 ferritic stainless steel specimens. The phase diagram of STS 430 steel obtained by calculation showed that the phase appeared at $1100^{circ}C$ showed as ${alpha}+{gamma}{ ightarrow}{gamma}{ ightarrow}{gamma}+Cr_2N{ ightarrow}{gamma}+Cr_2N+CrN$ with increasing nitrogen concentration. Also, the transformation of ${gamma}{ ightarrow}Cr_2N$ during heat treatment isothermally at $780^{circ}C$, nitrogen pearlite with lamellar type was fully formed at the nitrogen permated surface layer for 10 hrs. However, this transformation was not completed for 1 hr, resulting nitrogen pearlite plus martensite. The cold rolled specimen of isothermally transformed at $780^{circ}C$ for 10 hrs after high temperature gas nitriding decreased the layer thickness of nitrogen pearlite inducing the deformation of hard $Cr_2N$ phase. the dissolution rate of $Cr_2N$ phase increased rapidly with increasing cold rolling ratio. Specimens with the microstructure of nitrogen pearlite (isothermally transformed at $780^{circ}C$ for 10 hrs) were possible to cold rolling without crack formation. However, the mixed structures of nitrogen pearlite + martensite (isothermally transformed at $780^{circ}C$ for 1 hr) were impossible to cold deformation without cracking.