The phrase "a big milestone of" is correct and usable in written English.
It can be used to indicate a significant achievement or event in a particular context, often related to projects, personal goals, or historical events.
Example: "Completing the project ahead of schedule was a big milestone of our team's success this year."
Alternatives: "a significant achievement of" or "a major landmark of".
When you're in business school, you think of getting venture capital as a big milestone of success.
Apple's iOS App Store always seems to outshine its computer-based counterpart, but it seems the company's Mac App Store has just hit a big milestone of its own.
As the world rang in the Near Year this this week, NASA was looking forward to a big milestone of its own — nine years and counting on the surface of Mars for an overachieving Red Planet rover mission.
"Eighty years is a big milestone for the Festival of Arts," Sharbie Higuchi, the Festival's director of marketing, said in a statement.
Me and the girlfriend had decided to spend our vacation in Santo Domingo, a big milestone for me, one of the biggest, really: my first time "home" in nearly twenty years.
"This is a big milestone for Sony," said Chris Dring, editor of industry news site, MCV.
I love that you made my sons first haircut into a big milestone instead of just a routine hospital procedure.
Most business profiles are by firms in the U.S., Brazil, Indonesia, Russia and the U.K. "One million advertisers is a big milestone, particularly because of the scale and diversity of the businesses on Instagram," Instagram's VP of business, James Quarles, said in a phone interview.
Today's it's reached a big milestone: exactly 50% of Android phones are running 2.1, the latest officially released version of Android (2.2, or Froyo, went live for a small number of users last month, but has not officially been deployed on a wide scale).
Bernie Morgan, Chair of the Dreamland Trust, described it as a "big milestone in the life of Dreamland".
