To shrink a log file,

you should call sp_dboption

before using DBCC shrinkfile

(see the script below)

It's usually enough.

/*

shrink a log file

WARNING: change the database name and the logical name of the log file (found in sysfiles).

database: MyDB

log file: MyDB_Log

*/

USE master

go

EXEC sp_dboption 'MyDB', 'trunc. log on chkpt.', 'TRUE'

USE MyDB

go

DBCC SHRINKFILE ('MyDB_Log', TRUNCATEONLY )

USE master

go

EXEC sp_dboption 'MyDB', 'trunc. log on chkpt.', 'FALSE'

USE MyDB