Combine crab apples and water in a flat-bottomed kettle; bring to a boil. Reduce heat; cover and simmer 25 minutes or until fruit is soft. Strain fruit through a jelly bag or 4 layers of cheesecloth, reserving 1 quart juice. Discard pulp.
Return reserved juice and sugar to kettle; stir well. Bring to a rolling boil, stirring frequently. Boil until mixture registers 220° on a candy thermometer or until mixture sheets from a cold metal spoon. Remove from heat, and skim off foam with a metal spoon.
Quickly ladle jelly into hot sterilized jars, leaving 1/4-inch headspace. Cover at once with metal lids, and screw bands tight. Process jelly in boiling water bath 5 minutes.