The deDendence of the effective penetration depth A, on the external magnetic field in a high-Tc superconducter Y B ~~C Y O . , , haa baen measured by both dc and ac methods at liquid-nigtrogen temperature.It is found that 1 , increases rapidly with increasing external field and is 0.13'4.15mm in macroscopic scale.The Hcl of the specimen, about 800e at 77K here, can be determined more reliably by comparing dc h e with ac 1, . The difference between xedc and A , , , , and the jumping of Xedc may indicate the characters of superconducting glass.