About employee who come out at night I have a table like this:

EmployeeId       Checktime
1              2015-10-14 07:51:25.000
1              2015-10-14 18:56:47.000
1              2015-10-15 00:04:04.000


I want to get the minimum datetime and maximum datetime from this table. I want to give parameter for the program like this '2015-10-01 07:00:00' and '2015-10-31 05:00:00' and the minimum datetime and maximum datetime will show the result like this for all the month of that employee.

Heading    Minimum                 Maximum
-------------------------------------------------------------
16         2015-10-14 09:07:32.000 2015-10-15 18:02:20.000 
17         2015-10-14 08:55:56.000 2015-10-15 18:02:07.000 
18         2015-10-14 09:01:17.000 2015-10-15 11:37:10.000 
73         2015-10-14 07:51:25.000 2015-10-15 00:04:04.000 


And my parameter was '2015-10-14 07:00:00' and '2015-10-15 05:00:00' but I want to pass parameter of whole month '2015-10-01 07:00:00' and '2015-10-31 05:00:00' and get this way answer for whole month.
If I correct understand Your question this should work. I have provided a link to SQL FIDDLE:

declare @max_date datetime
declare @min_date datetime

select @min_date = '2015-10-01 07:00:00'
select @max_date = '2015-10-31 05:00:00'

SELECT EmployeeId
    , (SELECT TOP 1 CAST(CAST(CONVERT(DATE, CheckTime) AS VARCHAR(10)) + ' ' + CAST(MIN(CAST(CheckTime AS TIME)) AS VARCHAR(8)) AS DATETIME) FROM table_name AS t2 WHERE t2.EmployeeId = t1.EmployeeId GROUP By CheckTime ORDER BY CAST(CheckTime AS TIME) ) AS  MinDateTime
    , (SELECT TOP 1 CAST(CAST(CONVERT(DATE, CheckTime) AS VARCHAR(10)) + ' ' + CAST(MIN(CAST(CheckTime AS TIME)) AS VARCHAR(8)) AS DATETIME) FROM table_name AS t2 WHERE t2.EmployeeId = t1.EmployeeId GROUP By CheckTime ORDER BY CAST(CheckTime AS TIME) DESC ) AS MaxDateTime 
FROM table_name AS t1
WHERE CheckTime BETWEEN @min_date and @max_date GROUP BY EmployeeId;


SQL Fiddle - http://sqlfiddle.com/#!6/fd199/2
DATA:

1   2015-10-14 18:06:22
1   2015-10-28 23:06:40
2   2015-10-07 18:13:58
2   2015-10-14 11:14:15
1   2015-10-14 19:20:11
2   2015-10-08 10:20:34
3   2015-10-01 20:25:28
3   2015-10-30 10:26:04


RESULT:

1   2015-10-14 18:06:22.000 2015-10-28 23:06:40.000
2   2015-10-08 10:20:34.000 2015-10-07 18:13:58.000
3   2015-10-30 10:26:04.000 2015-10-01 20:25:28.000