ECCS (Emergency Core Cooling System) is one of the engineering safety systems in a PWR (Pressurized Water Reactor), which is used to supply coolant to the core in case of a LOCA (Loss Of Coolant Accident) in order to avoid the core meltdown. During the accident, steam-water two-phase flow occurs in the main loop and then an injection flow comes from the ECCS. The void fraction of the main loop two-phase flow with an injection flow is important to the safety of the reactor. Experiment on ECCS two-phase flow void fraction was conducted in Xi'an Jiaotong University, China. The gamma ray attenuation was combined with visual observation to measure the void fraction. The results showed that the combined method can measure void fraction accurately and observe the flow pattern directly. Four typical flow patterns occurred in two-phase flow with a jet flow and they were stratified flow, wave flow, slug flow and annular flow. Due to ignoring the condensation caused by a jet flow, several conventional correlations had a large deviation on predicting void fraction. Based on the present experiment data, a new correlation was fit to calculate the void fraction of two-phase flow with an injection flow.