1

In my thesis, I want to use the statement "Chapter one" instead of "Chapter 1". Do I add specific changes into the following commands?

 \titleformat{\chapter}[display]
{\vspace{5cm}\filcenter}
{\filcenter \LARGE\bfseries
\chaptertitlename\ \thechapter}
{1ex}
{\vspace*{5ex}\filcenter\Huge\bfseries}
[\vfill\clearpage]
1

You just have to ensure the the \ifcase specification in the \justwords macro goes out far enough to include the largest chapter number.

\documentclass{book}
\usepackage{titlesec}
\titleformat{\chapter}[display]
{\vspace{5cm}\filcenter}
{\filcenter \LARGE\bfseries
\chaptertitlename\ \justwords{\thechapter}}
{1ex}
{\vspace*{5ex}\filcenter\Huge\bfseries}
[\vfill\clearpage]
\def\justwords#1{\ifcase#1 \or one\or two\or three\or four\or etc.\fi}
\begin{document}
\chapter{ABC}

\chapter{DEF}
\end{document}

enter image description here

ADDENDUM

Thanks to Willy for pointing out the fmtcount package already has this feature in the form of the \numberstringnum macro.

\documentclass{book}
\usepackage{titlesec,fmtcount}
\titleformat{\chapter}[display]
{\vspace{5cm}\filcenter}
{\filcenter \LARGE\bfseries
\chaptertitlename\ \numberstringnum{\thechapter}}
{1ex}
{\vspace*{5ex}\filcenter\Huge\bfseries}
[\vfill\clearpage]
\begin{document}
\chapter{ABC}

\chapter{DEF}
\end{document}

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