Aftermarket auto parts supplier U.S. Auto Parts Network Inc. priced its initial public offering at $10 per share for 10 million shares Friday and saw immediate gains.

Shares in the Carson company immediately shot up 20 percent ($2 per share) to $12 but slowed by midday, gaining 15 percent to sit at $11.50.

The 10 million shares represent about one-third of the company, which has a market capitalization of $300 million.

Underwriters for the IPO include RBC Capital Markets, Thomas Weisel Partners LLC, Piper Jaffray & Co. and JMP Securities LLC. They have the option to buy up to 1.5 million additional shares at the IPO price to cover any overallotments.

The company sells parts through a network of Web sites and had reported sales of $59.7 million in 2005. Profit totaled $6.8 million last year.