Iwakuma, on the disabled list with an injured right middle finger, allowed three runs -- two earned -- on six hits, striking out two and walking none against Las Vegas.
Iwakuma (10-5) scattered four hits and allowed two runs over 7-1/3 innings, while Dickey (9-11) yielded just one run, despite giving up eight hits in 7-2/3 innings.
The 31-year-old Iwakuma went 9-5 with two saves and a 3.16 ERA overall in 30 appearances, including 8-4, 2.65 ERA in 16 starts. His ERA as a starter set a rookie club record, bettering the previous mark of 2.67 by Felix Hernandez in 2005.
Iwakuma (9-5) lowered his ERA to 2.65 in 16 starts to set a Mariners record for lowest ERA by a starting pitcher in his first big league season. Felix Hernandez set the previous record with a 2.67 ERA in 2005.
Iwakuma had pitched in 226 games over 11 Pacific League seasons for the Kintetsu Buffaloes and Rakuten Eagles but had been used primarily as a starter.