Twisted Alexander invariants of knots are well-defined up to multiplication of units. We get rid of this multiplicative ambiguity via a combinatorial method and define normalized twisted Alexander invariants. We then show that the invariants coincide with sign-determined Reidemeister torsion in a normalized setting, and refine the duality theorem. We further obtain necessary conditions on the invariants for a knot to be fibered, and study behavior of the highest degrees of the invariants.2010 Mathematics Subject Classification. Primary 57M25, Secondary 57M05, 57Q10. Key words and phrases. twisted Alexander invariant, Reidemeister torsion, fibered knot, free genus.(See Theorem 6.6.)This paper is organized as follows. In the next section, we first review the definition of twisted Alexander invariants for knots. We also describe how to compute them from a presentation of a knot group and the duality theorem for unitary representations. In Section 3, we review Turaev's sign-determined Reidemeister torsion and the relation with twisted Alexander invariants. In Section 4, we establish normalization of twisted Alexander invariants. In Section 5, we refine the correspondence with sign-determined Reidemeister torsion and the duality theorem for twisted Alexander invariants. Section 6 is devoted to applications. Here we extend the result of Cha [C], Goda-Kitano-Morifuji [GKM] and Friedl-Kim [FK] for fibered knots, and study behavior of the highest degrees of the normalized invariants to obtain (1.1).Note. This article appeared first in 2007 on the arXiv, and has remained long to be unpublished. Since then twisted Alexander invariants and Reidemeister torsion for knots and 3-manifolds have been further intensively studied by many researchers. We refer the reader to the survey papers [FV5, Mo] and the recent preprint [DFL] for details and references. As this article has been already referred in the papers [DFJ, DFV, FKK, FV2, FV3, FV4, FV5, FV6, FV7, KM, SW] and frequently suggested to be published, we think that it might be worthwhile to have it published.