Combine grape juice concentrate and water in a 2-quart glass measuring cup. Gradually add pectin, stirring well. Let mixture stand 5 minutes. Microwave at HIGH until mixture comes to a rolling boil, stirring after 4 minutes. Boil 1 minute. Remove from heat; skim foam if necessary. Add sweetener, stirring until sweetener is completely dissolved.
Pour into hot sterilized jars, leaving 1/4 inch at the top of jar; wipe jar rims. Cover at once with metal lids, and screw on bands. Process in boiling water bath 5 minutes.