VLBI Observations of Mkn 501 and Mkn 421

We present here two epochs of Space VLBI observations at 18 cm of the BL-Lac type object Mkn 501. Thanks to the high resolution of these new data we have found that the inner jet is centrally brightened at its beginning but becomes extended and limb brightened at ~ 8 mas from the core. Moreover a comparison between the two epochs shows the presence of a possible proper motion with apparent velocity = 6.7c. VLBI data at 6 and 18 cm of the BL-Lac type object Mkn 421 are also presented. Observational data have been used to constrain the jet velocity and orientation.