SQL server 2005 online indexing

Welcome to the forum!.
Clustering indexes are on line.
Can you post your query and all indexes in that table?

What about to create cluster index desc?
CREATE [ UNIQUE ] [ CLUSTERED | NONCLUSTERED ] INDEX index_name ON <object> ( column_name [ ASC | DESC ] [ ,...n ] ) [ WITH <backward_compatible_index_option> [ ,...n ] ] [ ON { filegroup_name | "default" } ]
because default is ASC so the cluster index is not useful in "order ...... desc"

[quote user="ismailadar"]
when i create index i select desc option but i wonder about how can i query whether my indexs online or offline?
[/quote]
Check out INDEXPROPERTY in BOL. One property is "IsDisabled". This is about the closest you can get. To the best of my knowledge there is no way to determine whether an index has been rebuilt online or offline. This is a rebuild option that you specify at runtime when the index is altered. Once this operation is completed, it is irrelevant.
Also, if your query takes "too long" for just 500 rows & you need this query frequently, it may worth considering a dedicated index for the query.

What kind of syntax you are using to perform ALTER INDEX ..>ONLINE operation?
As per BOL, if you use ALTER INDEX REBUILD the indexes that are excluded are: Disabled clustered index or disabled indexed view, XML index and Index on a local temp table. Specifying the keyword ALL may cause the operation to fail when the table contains an excluded index.
When you perform online index operations, the following guidelines apply:

• Clustered indexes must be created, rebuilt, or dropped offline when the underlying table contains large object (LOB) data types: image, ntext, text, varchar(max), nvarchar(max), varbinary(max), and xml.
  
  
  • Nonunique nonclustered indexes can be created online when the table contains LOB data types but none of these columns are used in the index definition as either key or nonkey (included) columns. Nonclustered indexes defined with LOB data type columns must be created or rebuilt offline.
    
    
    • Indexes on local temp tables cannot be created, rebuilt, or dropped online. This restriction does not apply to indexes on global temp tables.