In 1950, Koetsier became the first Kapellmeister of the Bavarian Radio Symphony Orchestra. As a composer, he wrote chamber music, and orchestral and choral works, as well as the opera Frans Hals. From 1966 to 1976, he taught conducting at the Hochschule für Musik und Theater München.
A well-known example of his composition is Brass Symphony from 1979, which is divided into three movements and arranged for a brass dectet.
In 1992, the Jan Koetsier Foundation was established by an endowment from the composer. One of the main responsibilities of the Foundation, which has been based at the Hochschule since 2001, is the organization of the biennial International Jan Koetsier Competition.
- Symphony No. 2 for Choir and Orchestra (1947)
- Partita for English Horn and Organ (1956)
- Concertino for Viola and Orchestra (1973)
- Introduction and Variations on a Theme from the Opera "Die Zaubergeige" by Werner Egk for Viola and Piano, Op. 82 No. 3 (1978)
- Duo Giocoso for Trumpet (or Oboe) and Viola, Op. 69 (1979)
- Concertino Drammatico for Violin, Viola and String Orchestra, Op. 88 (1981)
- Concertino for Tuba and Orchestra, Op. 77 (1978/rev. 1982)
- II. Duo Concertante for Violoncello, Bassoon and Orchestra, Op. 92 (1983)
- Franzosiche Konzert op.98 for two flutes and string orchestra
- Ballade voor althobo (English horn) en piano, Op. 90 (1981)
- II. Duo Concertante for Violoncello, Bassoon and Orchestra, Op. 92. MiX-5: Premiere Recordings for Bassoon and Cello. Lynne Feller-Marshall, bassoon; John Marshall, cello. 2011. CD. With music by Martin Zalba Ibanez, Mike Curtis, Guang Yu Liu, Paul Desmond, Max Stern, Arthur Frackenpohl, Paul Hindemith, Béla Bartók, and Jimi Hendrix.
- Franzosische Konzert, op. 98 for two flutes and string orchestra. World premiere. Flutes: Andras Adorjan, Marianne Henkel, 'Franz Liszt' chamber orchestra, János Rolla. CD with music by C.P.E Bach, Ignaz Pleyel.
- Jan Koetsier Foundation (Hochschule für Musik und Theater München)
- Obituary in Neue Musikzeitung (in German)