The emergency services were called to a stabbing soon after 1pm today (April 5) in Erconwald Street near Old Oak Common Lane.
Paramedics and officers arrived with an air ambulance car and treated two males at the scene.
One of them died at the scene despite efforts to save him and the second was taken to hospital.
One person has been arrested on suspicion of murder.
His condition of the victim is not known at this stage, nor are ages of those involved.
Erconwald Street remains closed while officers carry out investigative work.
A Metropolitan Police spokesperson said: “Anybody who witnessed the incident – or who has potentially relevant information – should contact the police on 101, quoting reference 3435/05APR.”
