I think you need:

select data1.month,sum(data1.qty) as qty,sum(data1.weight) as weight, sum(data1.amount) as amount,   
round(sum(amount)/iif(sum(weight)=0, null, sum(weight)),2)  as kgrt
	from
	(select	month(date) as month,qty,qty * weight as weight,cr_amount as amount from cashsalp
	where type='P' 
	union all
	select 	month(date) as month, qty,qty * weight as weight, cr_amount as amount from crsalp
	where type='P')
	as data1
	group by data1.month

It is always better to apply calculations at the very end to avoid problems with rounding and incorrect results.

>Dear..
>
>I am using this data
>
>

>select data1.month,sum(data1.qty) as qty,sum(data1.weight) as weight,sum(data1.amount) as amount,max(data1.kgrt)as kgrt
>	from
>	(select	month(date) as month,qty,qty * weight as weight,round(cr_amount/weight,2)as kgrt,cr_amount as amount from cashsalp
>	where type='P' 
>	union all
>	select 	month(date) as month, qty,qty * weight as weight,round(cr_amount/weight,2)as kgrt,cr_amount as amount from crsalp
>	where type='P')
>	as data1
>	group by data1.month
>

>
>In result, field kgrt is showing wrong data like
>
>144577602/4183502=34.55
>
>but is showing 40537.50
>
>Please help