Example : I have a running total field which name is "claimnovember"
   And in "evaluate", I use a formula:Month ({claims.date})=11
   If there is no match condition, then the content of   my "claimnovember"
will be blank.
 And if I have a formula field which name is "total" and "total" is
claimnovember + claimdecember, then the "total" will be blank although the
"claimdecember" is not blank. (because the claimnovember is blank)
   How if I'd like to set default value in "claimnovember" to 0 (zero), so
my total will not blank anymore.

Thank you very much.

Check for isnull with an if for each running total field as in

if isnull(claimnovember)
    then claimdecember
elseif isnull(claimdecember)
else
claimnovember + claimdecember

The alternative is to use formulas which evaluate nulls and use a summary of
that field.

If the problem is that you have null data in the records, you can set the
report oprions to convert nulls to default type.