It is well known that a large DNA aggregate induced by spermidine, namely macro-aggregate, has a liquid-crystal-like feature in a structural manner. However, nobody finds out that the aggregation condition as well as the pathway to the aggregate under nearly physiological conditions containing Mg2+, ATP, and their mixture. In this work, the formation of liquid-crystal-like DNA macro-aggregate under such conditions leas been investigated via measuring the DNA precipitation condition, or phase diagram, and also observing their morphology along the precipitation with polarizing microscopy. These obtained results, such as the DNA phase diagram and morphological variations of the aggregates, suggest not only the effects of the added ions except aggregating agent on DNA aggregation but also biological implications of the present aggregation.