The German added that Koscielny's anticipatory skills and quick pace are his best attributes and make him a difficult proposition for strikers.
Mertesacker also praised Koscielny for his performances for France at Euro2016.
"To watch him is a joy for me because he's my team-mate," Mertesacker told Arsenal Player. "He's done so well for his country.
"It's really fun to watch him. He did so well during the tournament and he's grown as a player during his career at Arsenal and with the national team, he's a figure there now. It took a bit longer for him to be such a big part of the French squad but now he's an integral part there.
"He's very good in a duel, one against one. He anticipates so well and he's so quick that there's no joy for strikers to play against him. He's so composed now and I think there's more to come from him next season.
"I'm really looking forward to seeing him again and seeing his strength on the pitch because he's still growing."
Both players have been playing together for the North London club ever since Mertesacker's arrival at Emirates Stadium in 2011.
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