  With the possible detection of the fastest spinning nuclear-powered pulsar
XTE J1739-285 of frequency 1122 Hz (0.8913 ms), it arouses us to constrain the
mass and radius of its central compact object and to imply the stellar matter
compositions: neutrons or quarks. Spun-up by the accreting materials to such a
high rotating speed, the compact star should have either a small radius or
short innermost stable circular orbit. By the empirical relation between the
upper kHz quasi-periodic oscillation frequency and star spin frequency, a
strong constraint on mass and radius is obtained as 1.51 solar masses and 10.9
km, which excludes most equations of states (EOSs) of normal neutrons and
strongly hints the star promisingly to be a strange quark star. Furthermore,
the star magnetic field is estimated to be about $4\times10^{7} (G) < B <
10^{9} (G) $, which reconciles with those of millisecond radio pulsars,
revealing the clues of the evolution linkage of two types of astrophysical
objects.
