NSString *str = @"hello";
NSData *data = [NSData dataWithBytes:str.UTF8String length:str.length];

以防Swift开发者来这里，

将NSString / String转换为NSData

var _nsdata = _nsstring.dataUsingEncoding(NSUTF8StringEncoding)

NSString* str = @"teststring";
NSData* data = [str dataUsingEncoding:NSUTF8StringEncoding];

NSString *str = @"Banana";
NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:true];

NSString *str = @"hello";
NSData *data = [NSData dataWithBytes:str.UTF8String length:str.length];

NSString *str = @"helowrld";
// This converts the string to an NSData object
NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding];

你可以从这个链接得到参考