Dear All            If we want a scale 1:20 then through scale command we only give 20so after this if we have line=100 its length will be 2000.But the question is that I want the scale 20 times to decrease.For this how much scale I will give and what is the formula to calculate this.Please guide me in this matter.

Very easy. The display of the dimension is control by 2 variables: Dimscale and Dimlfac. Dimscale is the scale of the text and arrows. The Dimlfac (dimline factor) control the dimline value. With Dimlfac = 20, the line of 100 length would be dim as 2000. With Dimlfac = 1/20 = 0.05, the line of 100 length would be dim as 5.Here is 2 useful lisps to control these 2 variables, avoiding create new dimstype each time you want to change their values:;1. Copy these following lisp lines (in blue text) to an empty lisp file. Save it in any name as you like;2. Load the file to AutoCAD (better put them in StartUp list if you find it is useful);In AutoCAD command line: type "dsc" for Dimscale setting, "df" for DimlFac setting command;Cheer :);DSC AND DF COMMANDS FOR ADJUSTING SILENTLY DIMSCALE AND DIMLFACT VARIABLES;++++++++ DF DIMLFAC++++++++++++(defun C:df (/ dimlf odf)  (setq odf (getvar "dimlfac"))  (setvar "cmdecho" 0)  (prompt (strcat "The current Dimlfac is " (rtos odf 2 5)))  (initget 128)  (setq dimlf (getreal "\nEnter DimlinearFactor <or press any keyboard to get dimension style from object>  "))  (if dimlf    (if (numberp dimlf)      (setvar "dimlfac" dimlf)      (restoreDimstyle)      )  )  (princ));end defun;++++++++DSC DIMSCALE+++(defun C:dsc (/ dimsc ods)  (setvar "cmdecho" 0)  (setq ods (getvar "dimscale"))    (prompt (strcat "The current Dimscale is " (rtos ods 2 5)))  (initget 128)  (prompt "\nDimscale standard value should be one of the following values: 1, 2, 5, 10, 20, 25, 50, 100, 200, 500, 1000, 2000 ...")  (Setq dimsc (getreal (strcat "\nEnter the Dimlinear Scale <" (rtos ods 2 5) "><or press keyboard to get the dimension style from object>  ")))  (if dimsc    (if (numberp dimsc)      (setvar "dimscale" dimsc)      (restoreDimstyle)      )    )  (setvar "LTSCALE" (getvar "DIMSCALE"))  (princ));end defun(defun restoreDimstyle (/ dimlf dimsc e el en)  (setq e (car(entsel)))  (if (null e)(exit))  (setq  el (entget e) en (cdr(assoc 0 el)) )  (if (or (= en "DIMENSION")(= en "LEADER"))    (progn      (command "-dimstyle" "restore" "" e)      (graphscr)      )    (prompt "No dimension selected")        )   (setq dimlf (getvar "dimlfac") dimsc (getvar "dimscale"))  (prompt (strcat "\nThe current Dimlfac is " (rtos dimlf 2 5)))  (prompt (strcat "\nThe current Dimscale is " (rtos dimsc 2 5)))  (princ)  );end defun

If I understood your question...
If you want to decrease a scale by a factor of 20, you have 2 ways you could enter this on the command line;
1- use a calculator, 1/20 =.05. So enter ".05" for the scale
2-You can enter ratios of whole nubers on the command line; so for the scale enter "1/20"
 
But....the golden rule of cad drawings is to draw ebverything real worls scale. Its's only when you put drawing to paper (or pdf) that you need a drawing scaled, so you handle this with plot scales, or better by plotting from paperspace, with a scaled viewport showing modelspace objects.

Thanks a lot for reply me and your reply is Ok,but my question is that if we have DWG in model space of different sections and these sections scale is 1:20 then we give scale factor in DIM type 0.400.So how it will come because I have a DWG like that.I will wait for your reponse.