\documentclass{article}
\usepackage{threeparttable}
\usepackage{multirow}
\usepackage{booktabs}
\begin{document}
\begin{table}[!ht]
\setlength\tabcolsep{0pt}
\centering
\begin{threeparttable}
\begin{tabular}{@{\extracolsep{2ex}}*{6}{lllccc}}
\toprule
 &  &  & \multicolumn{3}{c}{\textbf{parameters}} \\
\cmidrule{4-6}
 &  &  & T\textsubscript{max} & C\textsuperscript{max} & \begin{tabular}{@{}c@{}}One Line \\ Another Line\end{tabular} \\
\textbf{group2} & \textbf{group} &  & \multicolumn{3}{c}{\textbf{value}} \\
\midrule
\multirow[t]{2}{*}{1} & 1 & \textbf{mean} & 1 & 5 & 9 \\
 & 3 & \textbf{mean} & 3 & 7 & 11 \\
\multirow[t]{2}{*}{2} & 2 & \textbf{mean} & 2 & 6 & 10 \\
 & 4 & \textbf{mean} & 4 & 8 & 12 \\
\bottomrule
\end{tabular}
\end{threeparttable}
\end{table}
\end{document}