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The projection effect is one of the biggest obstacles in learning the real properties of coronal mass ejections (CMEs) and forecasting their geoeffectiveness. To evaluate the projection effect, 86 full halo CMEs (FHCMEs) listed in the Coordinated Data Analysis Workshop CME catalog from 1 March 2007 to 31 May 2012 are investigated. By applying the Graduated Cylindrical Shell model, we obtain the deprojected values of the propagation velocity, direction, and angular width of these FHCMEs and compare them with the projected values measured in the plane‐of‐sky. Although these CMEs look full halo in the view angle of SOHO, it is found that their propagation directions and angular widths could vary in a large range, implying projection effect is a major reason causing a CME being halo, but not the only one. Furthermore, the comparison of the deprojected and projected velocities reveals that most FHCMEs originating within 45° of the Sun‐Earth line with a projected speed slower than 900 km s −1  suffer from large projection effect, while the FHCMEs originating far from the vicinity of solar disk center or moving faster than 900 km s −1 have small projection effect. Thus, for the latter class of FHCMEs, it is not necessary to correct the measured velocities.

Full halo coronal mass ejections: Do we need to correct the projection effect in terms of velocity?