Active galactic nuclei (AGN) often exhibit powerful outflows or relativistic 
jets as a consequence of the material accretion onto the supermassive 
black holes. Although a number of theoretical and observational efforts have 
been made over the past decades, many of basic questions remain to be solved; 
how are jets formed, collimated, and accelerated?: how and where do jets 
produce the high-energy emission up to MeV, GeV and TeV?: are jets 
common phenomena in accreting black holes, or just a specific event only 
for active black holes? 
To address these issues, one of the promising approaches is to directly probe 
the immediate vicinity of the central engine with high-resolution VLBI 
technique. In this talk, I will present our intensive VLBI observations on two 
distinct sources; one is an active jet source M87 (Virgo A) while the other 
is a quiescent nuclei M104 (Sombrero galaxy). Thanks to their remarkable 
proximity and very large black hole, VLBI is able to access an exceptionally 
close vicinity of black holes for these two.   
For M87, my talk will include: (1) the jet launch, collimation structure down 
to ~10 Schwarzshild radii (Rs) revealed by multi-frequency VLBA observations: 
(2) an effort to locate the site of TeV gamma-ray production with multi-epoch 
EVN plus VERA observations. For M104, we for the first time revealed the 
nuclear structure of this source down to tens of Rs together with the 
discovery of a nuclear radio jet.