The first problem that comes up with this question is one of semantics. For example, many prefer "sin nature," others prefer "sinful nature," and still others prefer the ambiguous "flesh." Whatever the specific names used for the warring parties, what is relevant is that an ongoing battle rages within the Christian.
The second problem is the actual definition of "nature." How this significant word is defined determines how one sees the distinction between the “old man” and the “new man” and its relevant outworking in the life of the Christian. One way to view "nature" is to understand it as a "capacity" within a believer. Thus, the old man is interpreted as the former way of life, that of an unbeliever. In this sense, the Christian has two competing capacities within him—the old capacity to sin and the new capacity to resist sinning. The unbeliever has no such competition within; he does not have the capacity for godliness because he has only the sin nature. That’s not to say he cannot do “good works,” but his motivation for those works is always tainted by his sinfulness. In addition, he cannot resist sinning because he doesn’t have the capacity to not sin.
Read More: http://www.gotquestions.org/two-natures.html