>> BACKUP LOG myDatabaseName WITH TRUNCATE_ONLY
>
>The command completed successfully. But the size of log file
>on the disk is not changed.
>
>MyData_Data.MDF 62528kb
>MyData_Log.LDF 1110720kb
>
>Anyway, is it normal the log file is so huge?

Truncation does not physically reduce the size of the log file. For that, run

USE myDatabaseName
DBCC SHRINKFILE (myDatabaseName_Log)

The log file retains all transactions executed on your database unless you set options to control that, that's why it may be bigger than the actual database.

HTH