Resistice transitions in the presence of magnetic field have been studied on ultrathin YBCO films and YBCO/PrBCO superlattices. We found that: i)The lower critical magnetic field in parallel coniguration (H∥ab) is about 10 tesla for 5 unit cell thickness YBCO film, ii)When field is applied perpendicular to abplane, correlation length along the flux line is about 10 unit cell length. © 1991.