VLBI Structures of Extragalactic Radio Sources at High Frequencies

VLBI observations at high frequencies show that most of extragalactic radio sources have a nucleus-jet structure on pc or sub-pc scales. Especially for flat-spectrum compact radio sources, the milli-arcsecond components with apparent superluminal velocity are detected with high probability. Multi-frequency observations monitor the new jet component production. Especially for radio OVV, the jet components have a tight relation with correlated broad-band(from optical to γ-ray) flux outbursts. The spectrum index distribution in some radio sources has been obtained, which makes it possible to identify the VLBI cores of these sources.