Conventional testing of fracture toughness measurement for thin metallic sheets is not possible owing to the large plastic zone ahead of the crack tip. Hence, the critical-crack tip opening angle \((\hbox {CTOA}_\mathrm{c})\) has been considered to be the key parameter to evaluate resistance of a thin sheet against stable crack propagation. In the present work, attempt has been made to measure the \(\hbox {CTOA}_\mathrm{c}\) of interstitial free high strength steel (IFHS) with 1 mm thickness using compact tension C(T) specimen. A high speed camera has been used for monitoring and imaging the progress of the crack length and crack tip angle during loading. The CTOA\(_\mathrm{c}\) measured from these photographs found to be \(12^{\circ }\,\pm \,1.5^{\circ }\). In these experiments, the crack tip velocity was also varied from 1.9 to 960 mm/min and no changes in the \(\hbox {CTOA}_\mathrm{c}\) value were observed. The high ductility of IFHS steel along with prevailing plane stress condition might be the reason of appreciably large \(\hbox {CTOA}_\mathrm{c}\) value.