A series of pseudo-quinary Ti20Zr20Hf20Be20(Cu20-xNix) (x=2.5, 5, 7.5, 10, 12.5, 15, 17.5 and 20at.%) high entropy bulk metallic glasses (HE-BMGs) with large glass forming ability (GFA) were successfully prepared by copper mold tilt-casing. The critical diameters (Dc) of these HE-BMGs are all above 12mm. In particular, the developed Ti20Zr20Hf20Be20(Cu7.5Ni12.5) and Ti20Zr20Hf20Be20Ni20 high entropy alloys (HEAs) can be produced in the amorphous state with diameters up to 30mm and 15mm, respectively, which are the largest HE-BMG and quinary HE-BMG hitherto. The two HE-BMGs also exhibit high yield strength, together with the plastic strain values of (3.0±1.1) % and (4.0±0.9) %, respectively. With increasing Ni additions in the pseudo-quinary HEAs, the crystallization growth resistance and thermal stability have been improved, which is apparently due to the sluggish diffusion of the atoms in the HEAs. Display omitted •These high entropy bulk metallic glasses (HE-BMGs) have large glass-forming ability.•These HE-BMGs have a critical diameter up to 30mm.•This is the largest size in the HE-BMGs reported so far.•All the developed HE-BMGs exhibit good mechanical properties.•The developed HE-BMGs possess high thermal stability.