IOS8 introduce a new concept named Configuring Content Margins , a new property named layoutMargins is also introduced , for the details of the property , please refer to the Apple Doc . The type of layoutMargins is UIEdgeInsets , by default the value is {8,8,8,8} . To remove the seperator line of TableView in IOS8 , in addition to set tableView.seperatorInset = UIEdgeInsetsZero , you must also do as  :
First define the macro
#define isIOS8SystemVersion (NSFoundationVersionNumber > NSFoundationVersionNumber_iOS_7_1)
In the UITableViewDelegate method add :
    - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
    {
         static NSString *reuseId = @"cellReuseID" ;
         UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:reuseId];
         if(!cell){
             cell = [[UITableViewCell alloc]initWithStyle:UITableViewCellStyleValue1 reuseIdentifier:reuseId];     
         if(isIOS8SystemVersion){   
             cell.layoutMargins = UIEdgeInsetsZero;
             cell.preservesSuperviewLayoutMargins =NO ;
         }
    }
Doing these will remove the seperator line . You can also do as follow :
    UITableView *tableView = [[UITableView alloc] init];
    if(isIOS8SystemVersion){
         tableView.layoutMargins = UIEdgeInsetsZero ;
    }
and in the UITableViewDelegate method add :
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
     static NSString *reuseId = @"cellReuseID" ;
     UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:reuseId];
     if(!cell){
         cell = [[UITableViewCell alloc]initWithStyle:UITableViewCellStyleValue1 reuseIdentifier:reuseId];     
     if(isIOS8SystemVersion){   
         cell.layoutMargins = UIEdgeInsetsZero;
     }
}