  We investigate magnetic properties of an $S$=1/2 quasi-one dimensional
organic antiferromagnet, D-F$_{5}$PNN using magnetization measurements taken at
temperatures as low as 0.5 K. Three distinct phases were observed consisting of
uniform, dimerized (D), and incommensurate (I) spin structures in the magnetic
field versus temperature plane, where a significant hysteresis appears between
D-I transitions in the field scan measurements. A combination of magnon ($S$=1)
and soliton ($S$=1/2) excitations have successfully reproduced the observed
magnetic susceptibility. In addition, such excitations provide a reasonable
interpretation of the temperature dependent electron spin resonance (ESR)
spectra. By comparison with the theoretical study, we conclude that
D-F$_{5}$PNN is an ideal compound for investigating the spin-Peierls
transition.
