Researchers at the University College London (UCL) and University of Milan-Bicocca also found that magnifying the body portion - the hand in case of an injection - to make it appear larger cut pain levels further still.
The study, the researchers said, sheds light on how the brain processes pain and a better understanding of this could lead to new treatments, the BBC reported.
For their study, which was funded by the Biotechnology and Biological Sciences Research Council (BBSRC), the research team recruited 18 volunteers.
They applied a heat probe to each participant`s hand, gradually increasing the temperature. As soon as this began to feel painful, the probe was removed and the temperature was recorded.
Patrick Haggard, professor of cognitive neuroscience from UCL, explained: "This gives us a measure of the pain threshold, and it is a safe and reliable way of testing when the brain pathways that underline pain become active."
The scientists then used a set of mirrors to manipulate what the volunteers saw.
They found that volunteers could tolerate on average 3C more heat when they were looking at their hand in the mirror, compared with when their hand was obscured by a block of wood.
Professor Haggard said: "You always advise children not to look when they are having an injection or a blood sample taken, but we have found that looking at the body is analgesic - just looking at the body reduces pain levels.
"So my advice would be to look at your arm, but try to avoid seeing the needle - if that`s possible."
In another experiment, the researchers used convex mirrors to enlarge the appearance of the participant`s hand. They found that doing so meant the volunteers were able to tolerate higher temperatures.
Conversely, when the team made the volunteers`hands look smaller, their pain threshold decreased.
The findings, published in Psychological Science, are helping to show how pain is processed in the brain, said the researchers.
The fact that pain levels were directly proportional to the size the body was viewed at was helping them to better understand the neurological basis of pain, they added.
Lead researcher Dr Flavia Mancini said: "Psychological therapies for pain usually focus on the source of pain, for example by changing expectations or attention.