Try using SqlDbType and set it to decimal and also specify size and precision. Check first sample
http://msdn.microsoft.com/en-us/library/yy6y35y8.aspx

and also

http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlparameter.scale.aspx

>Yet another question on setting up values to parameters to be passed to a sql server stored procedure.
>
>Here is an example.
>One of the parameters of the stored procedure is of type number:
>
>

>@nTest numeric(10,2) OUTPUT
>--- set value
>set @nTest = 3333.33
>

>
>Then in the C# code when creating the parameter I do the following
>

>param.DbType = System.Data.DbType.Decimal;
>param.Value = 0;
>param.Direction.InputOutput;
>

>
>But after I call the oCommand.ExecuteNonQuery() I get error
>
>

>Error converting data type numeric to numeric
>

>
>What am I missing?
>
>TIA.
>
>UPDATE: The title of the thread is wrong. It should have said, Using Decimal type parameter.