We present the results of EVN and MERLIN 5 GHz observations of nine ultra-high-energy synchrotron peak BL Lacs ( UHBLs ) selected as all BL Lacs with log ( \nu _ { peak } / Hz ) > 20 from Nieppola et al..
The radio structure was investigated for these sources , in combination with the available VLBA archive data .
We found that the core-jet structure is detected in five sources , while four sources only have a compact core on pc scale .
The core of all sources shows high brightness temperature ( with mean and median values log ( T _ { b } / { K } ) \sim 11 , which implies that the beaming effect likely present in all sources .
When the multi-epoch VLBI data are available , we found no significant variations either for core or total flux density in two sources ( 2E 0414+0057 and EXO 0706.1+5913 ) , and no evident proper motion in 2E 0414+0057 , while the superluminal motion is likely detected in EXO 0706.1+5913 .
Our sources are found to be less compact than the typical HBLs in Giroletti et al , by comparing the ratio of the VLBI total flux to the core flux at arcsec scale .
Combining all our results , we propose that the beaming effect might be present in the jets of UHBLs , however , it is likely weaker than that of typical HBLs .
Moreover , we found that UHBLs could be less Doppler beamed versions of HBLs with similar jet power , by comparing the distribution of redshift , and radio luminosities .
The results are in good consistence with the expectations from our previous work .