Heat transfer enhancement with respect to void fraction (i.e. vapor quality) is investigated in a flow boiling of R407C using twisted tapes in a tubular evaporator of 13.40 mm inner diameter and 2m length. Following operating conditions are considered for investigations: (i) heat flux – 1.5 to 9.5 kWm-2; (ii) mass flux - 150 to 300 kg s-1 m-2; (iii) evaporating temperature ranging from -150C to +150C; (iv) average void fraction( vapor quality) -0.05 to 0.90. Heat Transfer enhancement due to twisted tapes is found to be varied with respect to void fraction (vapor quality) and operating conditions. 10 to 40% void fraction is found to be most suitable range for heat transfer enhancement for all operating conditions. Twisted tapes of twist ratio 12, 10 & 8 are compared for their heat transfer performance. Twisted tape of twist ratio 12 has shown maximum heat transfer enhancement.