1 Liter is already 1 Kilogram :)

Assuming mL=cubic centimeters, 1000mL=1000 cubic centimeters

1000 cubic centimeters = 1000 x cm x cm x cm=1000 x 0.1 dm x 0.1 dm x 0.1 dm=1 cubic decimeter

1000mL=1 Liter (as you already know that)

Therefore, 1000 cubic centimeters = 1000 mL = 1 L = 1 cubic decimeter

I hope this answers your question :)