Let $f$ and $g$ be both continuous functions on $\left( 0,\infty \right) $ with $g\left( t\right) >0$ for $t\in \left( 0,\infty \right) $ and let $ F\left( x\right) =\mathcal{L}\left( f\right) $, $G\left( x\right) =\mathcal{L }\left( g\right) $ be respectively the Laplace transforms of $f$ and $g$ converging for $x>0$. We prove that if there is a $t^{\ast }\in \left( 0,\infty \right) $ such that $f/g$ is strictly increasing on $\left( 0,t^{\ast }\right) $ and strictly decreasing on $\left( t^{\ast },\infty \right) $, then the ratio $F/G$ is decreasing on $\left( 0,\infty \right) $ if and only if \begin{equation*} H_{F,G}\left( 0^{+}\right) =\lim_{x\rightarrow 0^{+}}\left( \frac{F^{\prime }\left( x\right) }{G^{\prime }\left( x\right) }G\left( x\right) -F\left( x\right) \right) \geq 0, \end{equation*} with \begin{equation*} \lim_{x\rightarrow 0^{+}}\frac{F\left( x\right) }{G\left( x\right) } =\lim_{t\rightarrow \infty }\frac{f\left( t\right) }{g\left( t\right) }\text{ \ and \ }\lim_{x\rightarrow \infty }\frac{F\left( x\right) }{G\left( x\right) }=\lim_{t\rightarrow 0^{+}}\frac{f\left( t\right) }{g\left( t\right) } \end{equation*} provide the indicated limits exist. While $H_{F,G}\left( 0^{+}\right) <0$, there is at leas one $x^{\ast }>0$ such that $F/G$ is increasing on $\left( 0,x^{\ast }\right) $ and decreasing on $\left( x^{\ast },\infty \right) $. As applications of this monotonicity rule, a unified treatment for certain bounds of psi function is presented, and some properties of the modified Bessel functions of the second are established. These show that the monotonicity rules in this paper may contribute to study for certain special functions because many special functions can be expressed as corresponding Laplace transforms.

24 pages