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Levels of violence at a young offenders institution dropped after teenagers were given sweets for good behaviour, inspectors found.
An inspection at YOI Feltham in West London found staff had adopted a new "mindset" focusing on incentives rather than punishment.
An earlier report found boys were kept in their cells for "too long" and sanctioned for "petty" misdemeanours.
The Ministry of Justice (MoJ) said it was "encouraging".
Feltham A holds 140 boys aged 16-17.
Dr Phillip Lee, of the MoJ, said: "It's encouraging to see the progress Feltham has made under very challenging circumstances and I am determined to see similar improvements across the youth estate."
In 2017, inspectors said the "punitive" regime at Feltham had contributed to a cycle of violence.
It also found the prison was "not safe for either staff or boys" after finding levels of "very serious" violence had risen.
However, the latest report said there had been a "dramatic" improvement in safety, with assaults on staff down by 80%.
The report highlighted the decrease in violence was partly due to a new "behaviour management philosophy", under which boys who behave well are given "immediate rewards".
The boys can select confectionery items from a "merit shop".
Another difference addressed in the report was that teenagers at the centre were now allowed to eat some of their meals together - previously they ate alone in their cells.
Commenting on the recent findings, Peter Clarke, HM Chief Inspector of Prisons, said: "Last year we reported how the focus had been on sanctions and regime restrictions, there was a cycle of violence and punitive responses, with no obvious strategy in place to break it.
"This had changed, and we found a new focus on rewards and incentives for good behaviour."
Inspectors noted, however, that a large number of children at Feltham did not always receive the support to which they were entitled from local authorities, in particular in ensuring suitable accommodation on release had been secured.
The report also urged the prison to tackle a problem with some boys getting into debt through gambling.