Resistive 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 parallel-to ab) is about 10 tesla for 5 unit cell thickness YBCO film, ii) When field is applied perpendicular to ab-plane, correlation length along the flux line is about 10 unit cell length.