  Let $\Xi(t)$ be a function relating to the Riemann zeta function $\zeta (s)$
with $s = \frac{1} {2} + it$. In this paper, we construct a function $v$
containing $t$ and $\Xi(t)$, and prove that $v$ satisfies a nonadjoint boundary
value problem to a nonsingular differential equation if $t$ is any nontrivial
zero of $\Xi(t)$. Inspecting properties of $v$ and using known results of
nontrivial zeros of $\zeta (s)$, we derive that nontrivial zeros of $\zeta (s)$
all have real part equal to $\frac{1} {2}$, which concludes that Riemann
Hypothesis is true.
