\documentclass[a4paper]{article}
\pagestyle{empty}
\usepackage{booktabs,multirow}
\begin{document}
  \begin{center}
    \begin{tabular}{lccccc}
      \toprule
      \multirow{2}{*}{aa} & \multicolumn{5}{c}{$d/\mathrm{cm}$} \tabularnewline
      \cmidrule(lr){2-6}
      & A & B & C & D & E\tabularnewline 
      \midrule
      A & 1.20 & 2.30 & 2.20 & 2.10 & 1.11 \tabularnewline  
      A & 1.20 & 2.30 & 2.20 & 2.10 & 1.11 \tabularnewline  
      A & 1.20 & 2.30 & 2.20 & 2.10 & 1.11 \tabularnewline  
      \bottomrule
    \end{tabular}
  \end{center}
\end{document}
