Combine lychee liquid with mango nectar. This should equal 2 cups.
In a small bowl, sprinkle gelatin over 1/4 cup juice; let soften 2 to 3 minutes.
Meanwhile, heat sugar with another 1/4 cup juice in a small saucepan over medium-high heat until dissolved. Remove from heat; stir in softened gelatin until dissolved, then stir in remaining 1 1/2 cups juice.
Place lychees in a 4-by-8-inch (6-cup-capacity) loaf pan; pour gelatin mixture over, pressing lychees gently to submerge completely.
Refrigerate until firm, at least 4 hours.
To remove from pan, run hotwater over base of pan for about 5 seconds. Invert onto a serving platter, and shake firmly to release. Slice to serve.
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